Device and method for operating a self-calibrating emissive pixel

ABSTRACT

Self-calibrating emissive pixel circuit, device and method for operating pixel. Method for operating includes: establishing sensor capacitor at predetermined starting voltage, delivering current to photon emitting device to cause photons to be emitted at predetermined target photon emission level, exposing sensor having electrical properties that vary according to photon flux on sensor to the emitted photon emission during at least portion of display frame time, permitting sensor capacitor to either charge or discharge from predetermined starting state through the sensor so that portion of frame time and resistance of sensor during portion of frame time determine amount of charge on sensor capacitor, measuring voltage or charge remaining on sensor capacitor at end of portion of frame time as indication of integrated photon flux and pixel luminance, and modifying image voltage and/or current applied to pixel during any subsequent display frame time using measured voltage as feedback parameter.

RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. 119and/or 35 U.S.C. 120 to U.S. Provisional Patent Application Ser. No.60/583,744 (Atty. Docket. No. 186051/US [474125-20]) filed Jun. 29, 2004naming as inventors Damoder Reddy and W. Edward Naugler, Jr., andentitled High-Impedance to Low-Impedance Conversion System for ActiveMatrix Emission Feedback Stabilized Flat Panel Display, whichapplication is incorporated by reference in its entirety.

This application is also related to the following applications, each ofwhich is hereby incorporated by reference: U.S. Utility application Ser.No. ______ (Atty. Docket. No. 186051/US/4 [474125-21]) filed 17 Dec.2004 and entitled System And Method For A Long-Life Luminance FeedbackStabilized Display Panel; U.S. Utility application Ser. No. ______(Atty. Docket. No. 186051/US/2 [474125-22]) filed 17 Dec. 2004 andentitled Feedback Control System and Method for Operating aHigh-Performance Stabilized Active-Matrix Emissive Display; U.S. Utilityapplication Ser. No. ______ (Atty. Docket. No. 186051/US/3 [474125-23])filed 17 Dec. 2004 and entitled Active-Matrix Display And PixelStructure For Feedback Stabilized Flat Panel Display; U.S. Utilityapplication Ser. No. ______ (Atty. Docket. No. 186051/US/5 [474125-25])filed 17 Dec. 2004 and entitled Method For Operating And IndividuallyControlling The Luminance Of Each Pixel In An Emissive Active-MatrixDisplay Device; U.S. Utility application Ser. No. ______ (Atty. Docket.No. 186051/US/6 [474125-26]) filed 17 Dec. 2004 and entitled Device AndMethod For Operating A Self-Calibrating Emissive Pixel, and U.S. Utilityapplication Ser. No. ______ (Atty. Docket. No. 186051/US/7 [474125-27])filed 17 Dec. 2004 and entitled High-Performance Emissive Display DeviceFor Computers, Information Appliances, And Entertainment Systems; eachof which applications is hereby incorporated by reference.

FIELD OF THE INVENTION

This application pertains generally to emissive flat panel displays andmore particularly to systems, devices and methods for making,calibrating, and operating emissive pixel flat panel displays to provideuniform light emission level and color over the surface of the displayinitially and throughout its operational life and to extend theoperational life of such displays.

BACKGROUND

Active matrix (AM) emissive displays and active matrix organic lightemitting diode (AMOLED) displays in particular rely on current levels inthe light emitting diode to produce luminance levels (light emissionlevel) in a matrix of pixels (picture elements). Each pixel is aseparate light emitting diode that is directly addressed and whereineach pixel has a sample and hold circuit so that a voltage can beapplied to the Organic Light Emitting Diode (OLED) display drivercontinuously over the duration of the frame.

The function of a flat panel display is to produce an image in variousshades of light and dark in correspondence to voltage levelsrepresenting the original image, or an image created by computersoftware. These light and dark shades may form or generate colors whenthey are rendered as different pixel types such as in red, blue, andgreen through the use of different colored emissive pixels or diodes orthrough the use of same colored or white pixels and filters. Sometimesthe set of three pixels used together to render a color by additivecombination of their respective photon flux are referred to assubpixels, but in the description to follow, little distinction is madebetween pixels and subpixels as the subpixels are pixels in their ownright and sets of pixels that are controlled as a set are merelycooperative sets of subpixels. Operation of sets of pixels or emittersto generate color are known in the art and not described in greaterdetail. The translation of the voltage image data into current generatedOLED photon emission (flux) levels presents several complex issuesinvolving the manufacture of the display and the aging of the displayduring operation and use by a user or consumer in the field.

In the case of a typical conventional OLED display, an image or datavoltage is placed on the gate of a power transistor (current source) inthe display pixel, which feeds and controls the amount or magnitude ofcurrent to the OLED pixel. The higher the gate voltage is, the higherwill be the current and therefore the brighter or more emissive will bethe pixel. Typically voltages (the signal data) supplied to thin-filmsemiconductor transistors (TFTs) having source, drain, and gateterminals are used to control the current to the pixel emitter elementsto render an appropriate gray level or pixel image luminance.

The circuits, methods of control, and even materials heretofore used inconventional implementations have significant limitations so that OLEDdisplay panel performance has suffered and has limited the applicationof such OLED technology for larger high-performance displays at consumeracceptable price.

A primary problem in such systems and devices is that it isconventionally extremely difficult if not impossible to produce uniformcurrent from pixel-to-pixel in a display using voltage image dataapplied to TFTs in this manner. This problem becomes particularly acuteas the displays become physically larger, have larger numbers of pixels,are driven to high current and luminance levels, and/or are operatedeither continuously or intermittently for longer periods of time (theyage). This problem arises at least in part because the current deliveredby a TFT at a particular gate voltage depends on many parameters, suchas for example the TFT threshold voltage, the effective electronmobility, and current gain of the TFT device (which may vary from TFTdevice to TFT device as a result of manufacturing variations,environmental exposure during operation, and/or operational history.These three parameters (threshold voltage, effective electron mobility,and current gain) may in turn depend, for example, on inter-grain andintra-grain trap densities, semiconductor thickness, andsemiconductor-to-gate dielectric trap densities. Other factors include:gate dielectric thickness, dielectric constant of the insulators, theTFT geometry, electron/hole mobilities, and other factors alone and incombination.

Among the problems at issue are how voltages (e.g. TFT voltages) to beapplied are determined and how that voltage is placed on the power TFTto give the right current level to produce the correct gray level. Somestudies have suggested a particular way or ways to use a particularluminance of a pixel to correct the voltage supplied to the pixel powerTFT (See for example, U.S. Pat. No. 6,518,962 B2 by Kimura and assignedSeiko-Epson; U.S. Pat. Nos. 6,542,138B1 and 6,489,631B2 assigned toPhilips, and the paper by Eko T. Lisuwandi at MIT (See “Feedback Circuitfor Organic LED Active-Matrix Display Drivers, by Eko T. Lisuwandisubmitted to the Department of Electrical and Computer Science inPartial Fulfillment of the Requirements for Degrees of Master ofEngineering in Electrical Engineering and Computer Science at theMassachusetts Institute of Technology, May 10, 2002). However, theseconventional attempts to improve OLED (or indeed other active emissiondisplay technologies) have not been entirely effective and are in oneway or another flawed.

For example, U.S. Pat. No. 6,542,138 B1 (assigned to Philips) describesa method that at most attempts to make pixels tend to be uniform to someextent over a frame duration but does not describe or suggest that exactemission levels corresponding to a series of gray levels can becontrolled. This invention described in this patent for example, uses alight sensitive discharge device across the signal hold capacitor thatmaintains the gate voltage on the OLED current driving TFT during theframe time. The photon emission from the OLED causes the light sensitivedischarge device to discharge the voltage on the holding capacitor thusturning off the current driving TFT and thus extinguishing the OLED. Therate of extinguishment is dependent on the level of photon emission;therefore if the pixel over-produces photon emission the OLED will beextinguished faster than were the pixel to under-produce photonemission. As a further refinement of such a system, the photosensitivedischarge device is a photo-transistor, the gate of which is controlledby the current passing through the OLED. The circuit is designed so thatat high current through the OLED the photo-transistor is in the offcondition because the voltage to the gate of the photo-transistor isclose to ground due to the high OLED current, but the photo-transistorwhile in the off condition acts like a reverse biased photo-diode andthe charge on the holding capacitor is slowly leaked to ground, causingthe current through the OLED to be reduced as the current is reduced.Due to the declining voltage on the storage capacitor the voltage riseson the gate of the photo-transistor. When the current decrease to acertain point the threshold voltage of the photo-transistor is exceededcausing the photo-transistor to turn on and dump the remaining charge inthe storage capacitor and thus shut off the OLED. The rapidity, andthus, the perceived luminance of the OLED is determined by the luminancelevel of the OLED. The higher the luminance of the OLED the faster isthe OLED shut off.

There are several objections to this approach. Firstly, the turning onof the photo-transistor to shut off the OLED depends on the thresholdvoltage of the photo-transistor. One of the problems that this approachis supposed to correct is the variable threshold voltages of the TFTsused in the pixel circuitry. This means that the time when the OLED isshut off will vary from pixel to pixel and thus actually contribute tothe non-uniformity between different pixels of the display. Secondly, atlow emission values the voltage applied to the gate of thephoto-transistor will be close to the threshold voltage at the beginningof the frame time. Any variations in threshold voltage are thereforegreatly magnified and the uncertainty of the actual luminance values isnot well controlled at all. Thirdly, the actual brightness perceived bythe viewer depends on the total photon emission during the frame. Thetotal photon emission during the frame depends at least in part on theinitial value of the data voltage supplied to the storage capacitor, therate of discharge of the storage capacitor during the off time of thephoto-transistor (which is dependent on the emission level of the OLEDcaused by the initial voltage), the threshold voltage of the currentcontrolling TFT whose gate is controlled by the voltage stored on thestorage capacitor, current gain of the current controlling TFT, theeffective electron mobility of the current controlling TFT, the agepoint of the OLED materials, the color spectrum of the OLED materialsand the threshold voltage of the photo-transistor. All these mentionedcontrolling parameters are not well controlled in the manufacturingprocess and therefore the pixel uniformity is not well controlled usingthe structures and methods of described or inferred by the U.S. Pat. No.6,542,138 B1 (Philips) reference.

U.S. Pat. No. 6,518,962 B2 by Kimura (assigned to Seiko-Epson) describescircuits in which current levels are obtained by certain pixelassociated sensors in the short address time allocated for making ameasurement. These are essentially instantaneous measurements and themeasurement time is too short to give a practically acceptablesignal-to-noise ratio so that useful information for determining thevoltage or current to be supplied to the TFT (or OLED pixel) can beextracted from the measurement. The signal extracted is expected to beon the order of a few nano-volts (10⁻⁹ volts) and the noise is expectedto be on the order of several volts due to the long conductor lineterminated essentially by an open circuit for a signal-to-noise ration(SNR) of less than about 0.1 percent Furthermore, it is also expectedthat different noise characteristics may arise for different regions ofa display owing to the different localized electromagnetic fields and tothe same pixels at different times.

Another limitation of Kimura et al (U.S. Pat. No. 6,518,962 B2) is thatthe system and method as described appears to apply a predeterminedsignal to the signal data line and it then alters this signal by thevoltage control unit to make the light level come close to the referencevalue. The predetermined data signal therefore appears to cause aluminance that is an incorrect luminance because it varies from thereference and is subsequently altered by the voltage-adjusting unit toproduce luminance that is only “close” to the reference. Kimuratherefore does not appear to actually match the reference or any othertarget luminance.

The work of Lisuwandi et al., which is generically and conceptuallysimilar to US U.S. Pat. No. 6,518,962 B2 has too long a feedbacksettling time (greater than 150 ms) and thus, is not practical,especially for displays that have dynamic content that changes fromframe to frame for normal computer screen, television, and similarapplications.

Conventional systems and methods that have attempted to control pixelluminance, have by-and-large attempted to measure instantaneous light orluminance levels that have been too small and too noisy to accuratelyand precisely provide such control. They have therefore been ineffectiveand their limitations will be even more severe as the size andperformance expectations of OLED displays increases.

These performance problems may likely be even more severe when amorphoussilicon (a-Si) is used for the display electronics. Amorphous silicon isthe semiconductor used by the LCD industry and has billions of dollarsinvested in the infrastructure. It is, therefore, desirable for themajor display manufacturers to use amorphous silicon. Early developmentof OLED active-matrix displays has employed the use of poly-silicon dueto its higher speed and better stability. There is very littleinvestment in poly-silicon infrastructure and the costs are high asopposed to amorphous silicon.

Recall that there are three forms of silicon conventionally used inelectrical integrated circuits. Crystalline silicon used in monolithicintegrated circuits (ICs). This type of silicon has no grain boundariessince the material is a solid crystal. This type of silicon (x-Si) hasonly one area for electrical charge to accumulate, and that area is atthe interface between the gate dielectric and the silicon surfacecontacted by the dielectric. The area of this interface is just thewidth and length of the gate dimensions.

Poly-silicon (p-Si) is made up of course grains of silicon having moreor less intimate contact with each other. In order for electrons to gofrom grain to grain and thus, travel through a p-Si channel in a fieldeffect transistor (FET), a certain amount of energy must be added. Also,the interface between grains can collect stray charges (both positive(holes) and negative (electrons) stray charges) just like the interfacebetween the dielectric and the silicon crystal in the x-Si material, butnow the area has greatly expanded. The intergranular area in the p-Si isinversely proportional to the grain size. Therefore, the smaller thegrain size, the greater the interfacing area will be and the greater thechance for stray charges to build up.

In the case of amorphous silicon (a-Si) the grain boundary area ismagnitudes greater than for p-Si. Trapped charge is normally thedominant characteristic that determines electron mobility and thresholdvoltage for a-Si devices and therefore any changes in the charge densityat the inter-grain boundaries causes fluctuation in the electronmobility and threshold voltage with much greater effect in the amorphoussilicon (a-Si) as compared to the poly-Silicon (p-Si) or crystallinesilicon (x-Si).

As display size increases, there is great desirability to use amorphoussilicon rather than poly-silicon or crystalline silicon. However, due tothe differences and fluctuations in electron and hole mobilitycharacteristics, stray electrical charge accumulation characteristics,and threshold voltage characteristics, it is increasingly difficult tomaintain a desired and uniform display luminance characteristics over alarge display surface at any single moment in time and as the displaydevice is used with amorphous silicon.

Various attempts have been made to overcome the uniformity problem inemissive displays, including some that have involved circuit-based, someof which are still in use today. These attempts have not been entirelysuccessful and do not meet the needs and application requirements of thecurrent and next generation of emissive display applications,particularly OLED display applications.

One scheme attempts to control photon emission by using a so called“current mirror” at the pixel, rather than using image voltages to driveor control the current through the OLED and hence control the OLED pixelluminance. Image currents are used in an attempt to force a luminancelevel current through the power TFT that feeds the OLED.

Another scheme compensates for TFT threshold variation by providing acircuit that determines the power TFT threshold voltage and then addsthe TFT threshold voltage to the image data voltage thus compensatingfor the threshold voltage so that variations or changes in the TFTthreshold voltage do not result in variation of the current supplied tothe OLED pixel luminance

These circuit based schemes are complex and expensive to produce andhave not been entirely satisfactory in maintaining pixel luminanceuniformity, because they do not compensate for the OLED materialdegradation, but only certain limited variations in the TFT.

It may be appreciated that for some devices in which OLED or otheremissive pixels are employed, the cumulative pixel on-time may berelatively short as compared to the age of the device carrying thedisplay, such as cell phones and personal data assistant (PDA) devices,because the display is normally on only when there is an active call oruser interaction. By comparison, an OLED display for a flat paneltelevision may be on and displaying a dynamically changing image forfive to ten hours a day. The requirements for luminance and coloruniformity are also greater for the television which must renderaccurate continuous tone images as compared to a small cell phonedisplay which may acceptably provide luminance uniformity and coloraccuracy at considerably lower levels.

It is known in the art that OLED displays that use different materialsfor the red emitter, green emitter, blue emitter of a three colorsubpixel set, will age or degrade at different rates so that after aperiod of operation such pixels in the displays (without correction)will have an observable color offset or shift that may depend on pixelluminance value. It may also be appreciated that as the color andluminance change will be specific to the individual pixel (subpixel) andoverall or global change to a particular color channel drive circuitwill generally be ineffective unless the cumulative effect on each pixelis the same.

Other schemes attempt to achieve a measure of uniformity by making acorrection based on a comparison of a measured pixel luminance to areference luminance. One scheme of this type has already be discussedrelative to U.S. Pat. No. 6,518,962 B2 by Kimura and assigned toSeiko-Epson. According to this scheme as described in the patent, thebrightness of the pixel is measured and compared with the brightness ofa reference pixel brightness to generate a difference signal or value.(It is noted that although the term “brightness” is commonly used,brightness is a subjective measure and may require the consideration ofa human viewer to be interpreted, whereas luminance is an objectivemeasure.) The difference signal or value is then used to alter thesignal voltage that drives the TFT supplying current to the pixel withthe intention of adjusting the pixel brightness in order that the finalor “settled” brightness (really luminance) comes “close” to thereference value. This scheme has several problems and does not solve theuniformity problem. Three problems are paramount with this scheme: (i)pixel brightness (really luminance) variation or “ringing” beforestabilizing at a settled value, (ii) inaccuracy due to a lowsignal-to-noise level and noise, and (iii) insufficient resolution as aresult of lack of pixel isolation. These problems better understood byreviewing the structure of one of the Kimura pixel structures.

Kimura et al. (U.S. Pat. No. 6,518,962) shows (See Kimura FIG. 19) whatis described as a block diagram showing an entire arrangement of adisplay apparatus according to a twelfth embodiment his invention andincluding a circuit diagram of a pixel. This Kimura pixel circuitstructure 61 has been redrawn and relabeled as presented in FIG. 1A sothat an appropriate comparison may subsequently be made with anembodiment of the pixel circuit structure 62 of the present invention.It is noted that the photodiode D1 of Kimura is connected to the voltagesupply line for its voltage. This approach is problematic from at leastthe standpoint of pixel luminance stability and repeatability becausethe exact voltage on that voltage supply line depends on the currentbeing used by the lines nearer the voltage supply for that voltage,because all the pixels attached to the line (in the column) are drawingcurrent which drops the voltage on the line. This voltage drop dependson what pixels are turned on and to what level of current draw they areexperiencing. In other words, the voltage that drives each of the Kimurapixels are dependent on the image data presented for display at otherpixels of the display. It will also be noted that the Kimura pixel lacksany isolation of the thin film diode. This means that all the sensorphotodiodes in the column are contributing current to the sensor readline at the same time.

Again, this photodiode configuration and the pixel structure thatcontains it is problematic because there is no information as to wherethe current (or charge, or voltage) originates from. Reference to theoriginal FIG. 19 of Kimura suggests that all the sensor read lines gointo a shift register, and each line appears to be read in series(rather than in parallel) with the next one. Performing a serial readoperation for each line would have to done during the address time whichimplies an exceedingly fast read rate and would permit only a very shorttime to make the current measurement. Such short measurements aresusceptible to imprecision and the effects of noise and may generallysupport only a very small signal to noise ratio.

Other conventional approaches also fail to overcome conventionallimitations. A particular luminance level produces a photocurrent in thesensor, and the size or magnitude of the photocurrent is an indication(in some instances is proportional or directly proportional to) of theluminance (photon flux through the sensor). Either the current or avoltage created across a resistive element (such as a resistor) by thecurrent that is measured to identify the luminance.

First, the pixel luminance will “ring” or oscillate for a time aroundthe reference value before stabilizing and reaching a stable luminancepoint. This stabilization takes time, time is important, and more timethan allowed by the short address time (t_(A)) which for most displayapplications (such as OLED displays having an array in the range of640×480 pixels) is the display frame time (t_(f)) divided by the displaynumber of lines (N_(L)). For a relatively small 160×120 pixel displaysuch as may be used in a hand-held computer or information appliance,the address time is about 0.13 ms and for relatively larger 800×600pixel display such as may be used in a Lap-top computer the address timeis about 0.027 ms. By comparison, the time to stabilize (t_(s)) such afeedback system has been calculated by Eko T. Lisuwandi at MIT (See“Feedback Circuit for Organic LED Active-Matrix Display Drivers, by EkoT. Lisuwandi submitted to the Department of Electrical and ComputerScience in Partial Fulfillment of the Requirements for Degrees of Masterof Engineering in Electrical Engineering and Computer Science at theMassachusetts Institute of Technology, May 10, 2002) to exceed 100 ms.This settling time is therefore unacceptably long for practicalactive-matrix type displays. The problems and limitations described hereare typical of conventional closed-loop feedback systems and methods,where a parameter or value is measured, sensed, or read and the readingfed-back to a control means that changes the read parameter (or aparameter derived from it), and applies or otherwise uses the changedparameter for operation. In this particular display context, since forany display that displays changing display content, the frame rate mustexceed 30 frames per second to prevent flickering. For most displaysthat display moving images the frame rate is 60 frames per second (fps).The frame duration (reciprocal of fps) will be less than about 20 ms, aclosed-loop feedback control scheme such as described by Kimura cannotbe realized for displays operating with display content that changes atrates faster than about 6 to 8 fps, as do normal video speeds fortelevision, computer displays.

A second problem with this scheme is that the scheme relies on a directreading from the light sensors in the pixels by a current measurementcircuit physically located outside the display area (or off glass). Thecurrent measurement circuit conventionally needs to be physicallylocated outside the display area because integrating high speedcircuitry directly on the display glass has been to costly in yield lossand added expense to be practical at this time; so it has not beenmerely a design choice as to where it is located. These conventionaldevices have used a reverse biased PIN diode as the sensor. Due to thehigh impedance value of the sensor (typically between about 1000 MegOhmsand 1 MegOhm), noise picked up by the wires or conductors attached tothe sensor and subsequently to the measuring equipment off the glasswill seriously obscure accurate reading of the pixel luminance. Forexample, the sensed signal may be a signal voltage in the range of a fewmillivolts (mv) and the noise on this signal when it reaches themeasuring equipment may typically be in the range between about a fewmillivolts and about several volts. Since the pixel uniformityrequirement for a 8-bit grayscale display may be 0.4 percent, any noisegreater than that will prevent achieving the required uniformity. Since,the signal voltage is a few millivolts a noise level of millivolts tovolts far exceeds the signal to noise ratio (which can be no worse that1 to 1) required to make a measurement with any accuracy at all. Third,this scheme generally, and the particular approach described in U.S.Pat. No. 6,518,962 B2 (Kimura), does not describe and gives noconsideration for isolating the sensors for individual rows thus, alsofailing to isolate the reading of the sensors since all sensor readingsin display array column appear to be combined into one current that isconducted to the measurement circuit off the glass. All the pixels in acolumn are on in an active-matrix display (as opposed to apassive-matrix display where the rows are on only one at a time);therefore, since the sensor line travels vertically up the display allthe sensors in a column are connected to the sensor line for that columnand each pixel's sensor will contribute to the total current in thesensor line making it impossible to determine the current contributed byany one pixels.

Therefore there remains a need for system, device, method, and computerprogram and computer program products that solve the afore describedproblems and limitations in the prior art, including the problems ofsettling times for conventional closed-loop control, noise interference,and sensor isolation.

SUMMARY

Systems, devices and methods for making, calibrating, and operating flatpanel displays to provide uniform pixel and display luminance emissionlevels (sometimes referred to as brightness) and colors over the surfaceof the display initially and throughout the operational life of adisplay and to extend the operational life of such displays.

A stabilized feedback display system and method for maintaining uniformpixel luminances in a display device. System includes a display devicehaving a plurality of emissive picture elements (pixels) each formedfrom at least one electronic circuit device, a display driver circuitreceiving a raw input image signal from an external image source andapplying a corrected image signal to the display, a display luminancedetector generating at least one display device luminance value, and aprocessing logic unit receiving the at least one display deviceluminance value and communicating information to the display drivercircuit, the display driver circuit using this communicated informationto generate a transformation for generating the corrected image signalfrom the raw input image signal.

System and method for controlling luminance of pixel in display. Methodincludes storing transformation between digital image gray level valueand display drive signal that generates luminance from pixelcorresponding to digital gray level value; identifying target gray levelvalue for particular pixel; generating display drive signalcorresponding to identified target gray level based on storedtransformation and driving particular pixel with drive signal duringfirst display frame; measuring parameter representative of actualmeasured luminance of particular pixel at a second time after the firsttime; determining difference between identified target luminance andactual measured luminance; modifying stored transformation forparticular pixel based on determined difference; and storing and usingmodified transformation for generating display drive signal forparticular pixel during frame time following first frame time. Controlsystem and circuits for controlling the luminance of a picture elementor pixel in a display device.

System, device, and method for operating active-matrix emissive pixeldisplay device. Method includes storing calibration value for pixels andgray levels displayed by pixels in memory; storing transformation inmemory for transforming first representations of gray level values tosecond representations; receiving first gray level representations ofimage pixel gray level values; transforming first representations tosecond representations for each pixel; generating image data and controlsignals for driving pixels during present display frame time; generatingintegrated photon flux signal for pixels in display indicative ofintegrated photon flux during portion of present display frame time;comparing plurality of integrated photon flux signals with calibrationvalues on pixel-by-pixel basis and generating plurality of comparisonresults indicating difference; and identifying deviation for each pixeland directing change in stored transformation to be applied duringsubsequent time. System provides a gray level logic, calibration memory,a comparator, and pixel deviation logic.

An emissive pixel device having integrated luminance sensor and a methodof operating an emissive pixel device having an integrated luminance orphoton flux sensor. Device includes light or photon emitting device,drive circuit generating current to drive light emitting device topredetermined luminance corresponding to an image voltage and applyingdrive current to light emitting device during frame time, photo sensorthat exhibits change in electrical characteristic in response to changein incident photon flux disposed near the light emitting device tointercept measurable photon flux when light emitting device is inemitting state, charge storage device coupled with sensor foraccumulating or releasing charges and exhibiting capacitance charge andvoltage proportional to the charge at a time; and control circuitcontrolling charging and discharging of charge storage device inresponse to changes in electrical characteristics of sensor during atleast a portion of the frame time.

Self-calibrating emissive pixel circuit, device and method for operatingpixel. Method for operating includes: establishing sensor capacitor atpredetermined starting voltage, delivering current to photon emittingdevice to cause photons to be emitted at predetermined target photonemission level, exposing sensor having electrical properties that varyaccording to photon flux on sensor to the emitted photon emission duringat least portion of display frame time, permitting sensor capacitor toeither charge or discharge from predetermined starting state through thesensor so that portion of frame time and resistance of sensor duringportion of frame time determine amount of charge on sensor capacitor,measuring voltage or charge remaining on sensor capacitor at end ofportion of frame time as indication of integrated photon flux and pixelluminance, and modifying image voltage and/or current applied to pixelduring any subsequent display frame time using measured voltage asfeedback parameter.

Information appliance device and method for operating display associatedwith information appliance. Information appliance includes displaydevice comprising plurality of active-matrix pixels arranged astwo-dimensional array, each pixel including a photon emitter, emitterdrive circuit receiving input image data for each pixel and generatingpixel drive signal intended to produce a corresponding target pixelluminance during frame time, and emitter luminance sensor andmeasurement circuit that measures electrical parameter indicative ofactual luminance of each pixel over portion of measurement display frametime; and display logic coupled to display and receiving pixel luminancerelated electrical parameter for each pixel and generating correctionfor application subsequent time period to input image data for eachpixel based on difference between target pixel luminance and measuredpixel luminance. Photon emitter may be OLED, electroluminescent, plasmaor other emissive device in flat panel display. Information appliancemay include a television monitor, a television receiver, a CD player, aDVD player, a computer monitor, a computer system, an automobileinstrument panel, an aircraft instrument display panel, a video game, acellular telephone, a personal data assistant (PDA), a telephone, agraphics system, a printing system, a scoreboard system, anentertainment system, a domestic or home appliance, a copy machine, aglobal positioning system navigation display, a dynamic art displaydevice, and/or devices combining these devices and systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are illustrations showing a comparison between anexemplary conventional pixel structure and a structure of a pixelaccording to an embodiment of the invention.

FIG. 2 is an illustration showing an embodiment of a Steadylight™emissive pixel and display calibration and stabilization circuit.

FIG. 3 is an illustration showing a first embodiment of a feedbackcontrol system for operating an active matrix display device withindividual pixel sensor integrated flux detection feedback

FIG. 4 is an illustration showing an embodiment of a second embodimentof a feedback control system for operating an active matrix displaydevice with individual pixel sensor integrated flux detection feedbackand including a calibration memory and pixel deviation memory formodifying and controlling operation of a gray level logic unit.

FIG. 5 is an illustration showing an embodiment of a pixel sensor andintegrated photon flux detection and measuring circuit using a voltagesensing amplifier.

FIG. 6 is an illustration showing an embodiment of a pixel sensor andintegrated photon flux detection and measuring circuit using a chargeamp-trans-impedance amplifier.

FIG. 7 is an illustration showing a first embodiment of an active matrixpixel including emitter, sensor, and photon-flux integrator elements.

FIG. 8 is an illustration showing a second embodiment of an activematrix pixel including emitter, sensor, and photon-flux integratorelements.

FIG. 9 is an illustration showing an embodiment of a first calibrationprocedure that may be executed to calibrate an active matrix displayaccording to the invention during the display manufacturing process.

FIG. 10 is an illustration showing embodiment of a second calibrationprocedure that may be executed to calibrate an active matrix displayaccording to the invention after the display has been manufactured suchas during a first time boot-up or power-on.

FIG. 11 is an illustration showing an embodiment of a procedure foroperating a display according to embodiments of the invention.

FIG. 12 is an illustration showing an embodiment of an active-matrixemissive pixel display device incorporating features of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention is directed to systems, devices and methods formaking, calibrating, and operating flat panel displays to provideuniform luminance emission levels and colors over the surface of thedisplay initially and throughout the operational life of a display andto extend the operational life of such displays.

U.S. Utility patent application Ser. No. 10/872,268 (Atty. Docket. No.34133/US/2 [474125-8]) filed May 6, 2004 naming as inventors DamoderReddy and W. Edward Naugler, Jr., and entitled Method and Apparatus forControlling Pixel Emission (which application is incorporated byreference in its entirety) describes and teaches the value of sensorarrays to improve organic light emitting diode (OLED) or other emissivepixel image quality, increase display life, and lower manufacturingcosts. The innovations described in this patent application generateemission measurements utilizing photo resistors and or photodiodes andphototransistors to send voltage or current signals to data processingcircuits located off the display substrate.

In one of the circuits described therein and shown in FIG. 2, andreferred to as the Steadylight™ calibration and stabilization circuit 40(Steadylight is a trademark of Nuelight Corporation), a voltage ramp 55is placed on the source of thin film transistor TFT T1 41. The voltagefrom output pin P3 42 of voltage comparator VC1 43 is high so that TFTT1 41 conducts the voltage ramp to the gate of TFT T2 46 and storagecapacitor C1 47. This causes OLED D1 48 to emit light with increasingintensity, which causes the resistance 49 of optical sensor S1 50 tosteadily decrease. As the resistance of sensor S1 50 decreases thevoltage across ground resistor R1 51 steadily increases placing anincreasing voltage on pin P1 44 of voltage comparator VC1 43. At thebeginning of the addressing cycle a reference voltage 46 is placed onpin P2 47 of voltage comparator VC1. The reference voltage representsthe desired emission value from OLED D1 48. When the voltage on pin P247 reaches the same voltage as the reference voltage on pin P2 theoutput voltage on pin P3 42 switches from a positive “on” voltage to anegative “off” voltage, thus turning off TFT T1 41 and freezing thevoltage to the gate of TFT T2 46, and thus, freezing the emission fromOLED D1 48 at the desired emission level. One difficulty is that theresistance of optical sensor S1 50 is in the gig-ohm range causing thevoltage across ground resistor R1 51 to possibly fluctuate with anyvoltage noise near the circuit. One of the greatest source of voltagenoise comes from the digital processing circuitry used to process thedata from the optical sensor S1. The reason for this is that thecurrents required to produce significant voltage are typically verysmall in a high impedance circuit. Therefore, the impedance shouldadvantageously be confined to the location of the pixel before a noisefree measurement can be made.

The present invention now described provides device, system, method, andother means to overcome the limitations associated with conventionalactive matrix displays generally, with any emissive display type(including for example, electroluminescent devices, plasma emissiondevices, or any other controllable emissive device) more particularly,and with organic light emitting diode (OLED) displays in particular, byproviding a means to measure and track the photon emission or luminanceof a pixel (the integrated photon flux over a defined period of time)and to use that information to ensure that any degradation mechanisms,whether they be pixel driver circuitry degradation due to gate thresholddrift as in the case of amorphous silicon, or degradation of the OLEDmaterials themselves, is compensated.

It will also be appreciated in light of the description provided herein,that even when the emissive device is an organic light emitting diode(OLED) there are several types, including but not limited to smallmolecule OLEDs, polymer OLEDs (PLEDs), phosphorescent OLEDs (PHOLEDs),and/or any other organic light emitting diode constructed from anyorganic material in any combination of single or multiple layers oforganic materials and electrodes.

Among the advantages of the invention, the invention provides a systemand method for measuring the luminance or photon flux over time (thetime duration of the frame) and storing that information to be used at alater time by the display to maintain uniformity, color balance and toextend life. The use of a photon flux integrator (sensor S1 coupled witha capacitor C2 in a particular circuit configuration) reduces noisefound on feedback systems operating with instantaneous photo currentsand with instantaneous feedback to the voltage drive system.

Among the advantages of the present invention, this invention recognizesthat the instantaneous photocurrents generated by light emitted(actually the photon flux emitted) by the OLED material in a pixel aretoo small to be used for controlling the voltages on the pixel and thus,we devised an in-pixel photon flux integration circuit so that ratherthan trying to measure the instantaneous photon flux emitted by thepixel the invention provides a device that integrates that flux over thetime length of a display frame. This causes the random instantaneousnoise fluctuations in the photon flux to cancel out over the frame time.The invention also provides a system and display panel that utilizesthis pixel device structure, and methods for calibrating, controlling,and operating the display. The invention therefore overcomes theproblems associated with conventional systems and methods that haveattempted to control pixel luminance using low-magnitude, noisy, andfluctuating measured instantaneous light or luminance measurements. Thein-pixel nature of the integrated photon flux measurements alsocompensates for pixel device material and electrical characteristics,operating environment, and operating history.

In at least one embodiment of the invention, a particular luminancelevel produces a photocurrent in the sensor, and the size or magnitudeof this photocurrent serves as an indication of the luminance (photonflux through the sensor). In at least one embodiment of the invention,the photocurrent is proportional (linearly or nonlinearly) to theluminance, and in at least one embodiment the photocurrent is directlyproportional to the luminance, or linearly proportional within anacceptable non-linear error. In one embodiment, either the current or avoltage created across a resistive element (such as a resistor) by thecurrent that is measured to identify the luminance. In otherembodiments, voltage accumulated on charge storage devices, such ascapacitors, are measured to identify the luminance.

Embodiments of the invention are capable of maintaining a pixel photonflux within one gray level (higher orders of accuracy can be obtained ifthe bit level is increased—this is a matter of cost) of an absolutephoton flux reference level, and pixel-to-pixel photon flux uniformityto within the same accuracy, over the life of the display. The inventivesystem, device, and method are also capable of adjusting the integratedphoton flux level of each and every pixel element (and hence also thepixel color and display color balance) so that the life-time of adisplay can be extended (and/or so that the aging or degradation can becontrolled in a preplanned manner), in spite of the known degradationcharacteristics of OLED displays, over a relatively longer period oftime than in conventional systems and methods.

One convention associated with defining the life time of a display is touse the time from an initial time (t₀) when the luminance is maximum toa half-life time (t_(x)) which is the time when the luminance has fallento one-half of the initial luminance. Thus, if the display has a 10,000hour life time (time t_(x)) it conventionally means that the displaywill be one-half as luminant (or have one-half the luminance) as it wasin the beginning (at time t₀).

The inventive device, system, and method can actually extend thepractical lifetime of a display and display system by extending thelength of time to one-half of maximum luminance (by compensating for thedegradation that leads to one-half luminance). For example, theinventive device, system, and method may extend the period of time tohalf-life by a factor of 2, 3, 4 or more (to 2t_(x), 3t_(x), 4t_(x), ormore). In one embodiment, this is accomplished by programming thedisplay to permit a controlled degradation over time. Recall, that theinventive device, system, and method can actually compensate 100% (andof course for any lesser amount of degradation) for the degrading of theluminance, but the display will last longer if it is permitted to slowlydegrade. Achieving a 100% compensation requires that additional voltagebe available to apply to the gate of the OLED current driving TFT. Theavailable voltage determines just how long degradation can be fullycompensated. If, however, the aging is partially compensated the displaywill eventually reach half luminance, but is a longer time than anun-compensated display.

Uniformity as used here means that the normal or average viewer will notusually be able to visually detect an aberrant pixel luminance (whereluminance or more loosely “brightness” is used to describe thecharacteristic in some conventional systems) or integrated photon flux(as a particular manner of characterizing luminance according toembodiments of invention described in specification) difference or colordifference relative to other pixels in the display. In the context ofthe invention to be described, embodiments of the invention are able tomaintain a calibration so that no pixel is more than one-half gray levelfrom a reference level. In one embodiment, having 8-bit per pixel percolor data (256 levels of gray), the uniformity is maintained to at orbetter than one gray level or ±0.4 percent. This is a pixel quantizationlevel of the display calibration, wherein if the pixel is determined tohave a luminance or integrated photon flux that is different from thereference luminance or integrated photon flux, the system and methoddrive the pixel to the gray level luminance or integrated photon fluxnearest the reference. Other embodiments of the invention may quantizeat a finer level of calibration, but normally the human visual systemwill not detect a variation even at the one-half gray level differencein a video display.

Recall that “brightness” is a subjective term. Luminance is an objectiveterm that has physical meaning and actual physical units. The mostcommon actual physical units used units today being cd/m2 (candelas persquare meter), which is the so called ‘nit’. In the inventive device,system, and method, the sensor operates by intercepting photons andturning them into charge carriers (holes and electrons) making thematerial of the sensor a better conductor and thus having lowerresistance. In an embodiment of the invention, the lower resistance ofthe sensor drains the charge on a capacitor (C2). The amount of drainedcharge is directly proportional to the number of photons that strike thesensor during the frame time. That is, the photons are counted(integrated) during the frame time. This integrated photon count isquantifiable.

A numerical example is now presented, without benefit of rigorous theoryand by way of example to illustrate aspects of the operation ofembodiments of the inventive device, system. The capacitance of thesensor capacitor (C2) is in the Pico farad (pf) or 1×10⁻¹² Farad range.If the capacitor has a capacitance of 1×10⁻¹² Farad, and if capacitor C2drains from an initial voltage of 10 volts at the beginning of the frameto ending voltage or 4 volts at the end of the frame, then 6×10⁻¹²coulombs of charge has passed to ground through the sensor. (Actuallythe starting and ending voltages may be selected at any value, however,voltage magnitude values in the 1 to 10 volt range are typical.) Thecorresponding amount of charge is 6×10⁻¹² coulombs. This is equal toabout 37,745,000 electrons. Since it only takes 0.25 electron volts topromote an electron into the conduction band and each light photon hasan energy of about 2 to 3 electron volts (depending for example upon thephoton wavelength or energy) it can be calculated that a red photon hasthe ability to put about 8 electrons into the conduction band and a bluephoton has the ability to put about 12 electrons into the conductionband. This means that the 37,745,000 electrons would mean that about4,681,000 red photons hit the sensor during a 16.7 ms frame time or thatabout 3,121,000 blue photons hit the sensor in the same frame time. Theabove values and numbers are provided as examples so that the principlesmay be understood and are not provided as exact values determined viarigorous calculation. The actual promotion of electrons into theconduction band depends on many factors. Among the most important isquantum efficiency, which is the amount of photon energy that promoteselectrons into the conduction band versus the amount of photon energyconverted into heating the semiconductor material.

It may therefore be appreciated that the invention operates as a photonflux integrator for the capacitor, sensor, and frame durationintegration time. The photon flux is a flow of photons through a unitarea (the area of the sensor) and the total photon count is the photonflux integrated over the sensor area and over the frame or otherappropriate partial frame or other integration time. The invention alsoprovides isolation so that the measurement of parameters from one pixeldo not impact the measurement of parameters from another pixel.

The invention has several aspects that may be used separately or foroptimum effect in combination to provide a greater synergistic effect.Some of these are listed below, while others will be apparent from thedescription of the embodiments of the invention and from the drawings.

In one aspect, the invention provides a Feedback Control System andMethod for High-Performance Stabilized Active-Matrix Emissive Display.In another aspect, the invention provides an Active-Matrix Display andPixel Architecture for Feedback Stabilized Flat Panel Display. Inanother aspect, the invention provides a Method for Calibration of anActive-Matrix Display and Pixel. These three aspects in particular, mayadvantageously be combined so that a display panel having the inventivepixel and sensor architecture and circuitry may be operated with offdisplay glass (or other display substrate) circuitry such as anoff-display integrated circuits (ICs) to provide a uniform and stabledisplay system.

In still another aspect, the invention provides a High-Impedance ToLow-Impedance Conversion System For Active Matrix Emission FeedbackStabilized Flat Panel Display.

In even still another aspect, the invention provides a High-Impedance toLow-Impedance Conversion Circuit for Active-Matrix Display Pixel andSensor.

In another aspect, the invention provides a Structure for and Method ofDesign of a High-Stability Integrated Light Sensor for Use in FeedbackControl System and Method For Making Same.

In even another aspect, the invention provides Long-Life andHigh-Stability Feedback-Stabilized Amorphous Silicon PhotoconductorBased OLED Display.

In another aspect, the invention provides computer programs, computerprogram products, data structures and other computer constructs andmachines that may be embodied in tangible media or memory devices andeither executed within or stored on a computer or other processor orhardware, including a processor and processor coupled memory of eithergeneral purpose of special purpose computers.

These and other aspects and features of the invention will become clearin light of the description provided herein and the referenced drawings.

Attention is first directed toward a comparison of an embodiment of theinventive display pixel with a conventional pixel structure so thataspects of the inventive pixel may be appreciated prior to describingthe manner in which its operation is controlled. Then aspects of theclosed-loop feedback control system that may be used for the inventivedisplay and pixel structure and architecture as well as for otherdisplay and pixel structures. Then several exemplary pixel structures,each having an emitter and a sensor, are described that may be utilizedwith the inventive control system. A method for calibrating the sensorsto establish reference integrated photon flux levels is then describedas well as some design methodology for designing sensors that have anappropriate capacitance and dark and illuminated resistance to providethe desired operation and support the inventive calibration andoperational procedures and methods. Operation of the calibrated displayand electronics so that stable and uniform operation is maintained isthen described.

When the pixel is on and radiating it emits photons at a particular ratesuch that at any point in time there is an instantaneous luminance. Inthe prior art the “brightness” referred to that has been measured hasbeen the instantaneous brightness. As suggested in the Backgroundsection, one problem associated with conventional systems and methodshas been that the amount of photo power intercepted by a sensor in apixel is that the photon power has been so small that random and/ornon-random noise sources swamp out the instantaneous signal. This isparticularly problematic when the read time for the pixel is small andsuch problems are compounded when the read signal from one pixel cannotbe distinguished from other pixels. Note that power is the time rate ofenergy and power is an issue in the prior-art as compared to the instantinvention. Photon flux and luminance are more-or-less interchangeableterms in that both of these terms are power terms.

With further reference to FIG. 1A, recall that the Kimura et al. pixelstructure connects photodiode D1 to the voltage supply line for itsvoltage, and that this approach is problematic because the exact voltageon that voltage supply line depends on the current being drawn by allthe other pixels attached to the line in the same column. There is avoltage drop that depends on the on-off state and gray level value ofother pixels in the display column. Recall also that a Kimura pixellacks any isolation of the photodiode TFTs for different pixels. Thismeans that all the sensor photodiodes in the column are contributingcurrent to the sensor read line at the same time and individual pixelsensor values cannot be determined. Finally, recall that the Kimurapixel and display configuration permits only a very short time to make acurrent measurement (the current measurement is essentiallyinstantaneous) and that instantaneous measurements are imprecise due tolow power, low signal strength, and high noise levels.

By comparison, the embodiment of the inventive pixel in FIG. 1Bovercomes at least these problems. The inventive device, system, andmethod solve the problems associated with delivery of voltage to theemissive diode, the sensor isolation problems, and the noise and lowpower problem. The structure and operation of this pixel and others aredescribed in detail elsewhere in this specification.

The photon flux integrator operates to store the energy (which is theintegral of power) delivered by the OLED to the sensor in a capacitor.What this means is that a weak photon flux is integrated over time for aduration of the display frame time, for example the photon energy isintegrated for 16.7 milliseconds (16.7 ms) or 16700 microseconds (16700μs). In conventional devices and systems, the energy is measured over aportion of the row address time, which is typically about 5 microseconds(5 μs). This means that in the inventive device, system, and method, thepower of the signal has been magnified by 16.7 ms divided by 5 μsmicroseconds for a gain factor of about 3,333 times. This represents again of about 35 db.

Furthermore, while the signal-to-noise ratio is greatly increased by the35 db of gain, the random noise is effectively cancelled because onaverage during this lengthy integration time there may generally beexpected to be substantially as many positive noise contributions asnegative noise contributions of the same magnitude. By integrating thesignal over time random noise is cancelled. These are significantadvantages over conventional systems and methods which require andtherefore attempt to obtain an accurate measurement of instantaneousluminance, however, they do not succeed in obtaining an accuratemeasurement because the signal to be detected is always plus or minusthe random noise and the magnitude of the random noise is at leastcomparable to the magnitude of the signal to be measured. If, inaddition, the photo-sensor employed in the pixel has an impedance in oron the order of the gig-ohm (10⁹ ohm) or greater range, the voltagenoise can be at the volt level, which would be a thousand times greaterthan the signal.

As described elsewhere in the detailed description, an additionaldifference between the inventive device, system, and method as comparedto conventional schemes is that the invention no longer attempts tocontrol the luminance of a pixel during the identical pixel write timeor cycle. In fact, in embodiments of the invention, the integratedphoton flux that is determined as an indication of the pixel luminanceduring one display frame time (or a portion of the display frame time)is used to control the integrated photon flux (and by extension thepixel luminance) during some subsequent display pixel frame time (orportion of such pixel display frame time). In one embodiment, thesubsequent display time is the next frame time or a portion thereof,while in other embodiments it is any future display time, such as a timethat is an integer multiple of frame times for that pixel, or asubsequent time that is triggered by an event such as by display poweron. Therefore, although the control and adjustment may appear to bereal-time and be indistinguishable to the display user (e.g. may lag bya frame time such as a 16.7 ms frame time) from a real-time feedbackbased measurement and control, some interpretations would suggest thatit is not real time. On the other hand, the measurement in one framewrite cycle and the use of the measurement to generate the pixel drivesignal is in the next frame write cycle are sufficiently close in timesuch that other interpretations may consider such operation to bereal-time or near-real time. Where many minutes, hours, or days wereallowed to pass between the measurement of integrated photon flux andadjustment of the pixel drive signal to take the measurement intoaccount, then the device, system, and method are less likely to becharacterized as real-time.

It will be appreciated that as OLED pixels (and other active photon orluminance emitters) used in displays my typically change over tens orhundreds of hours from a previous operating characteristic, once aparticular pixel has been adjusted, the need to update a pixel's drivecharacteristic every frame diminishes. Therefore, performing themeasurement and adjustment every frame is not normally necessary.

1. Exemplary Control System and Method Description

An exemplary display system 200 is illustrated in a first embodiment ofa feedback control system of FIG. 3 and includes two primary components,a display device 201 having a plurality of picture elements or pixels202 and photon flux integrator circuits 203, and display driver andcontrol electronics (optionally including software and/or firmware) 204to drive and control the display device 201. The drive and controlelectronics are responsible for converting image data 205 into theappropriate pixel drive and control signals 206 to the pixels 202 sothat their apparent gray level or integrated photon flux (and theircolor for a color display) within the image is correct or match acommanded integrated photon flux and color. It will be appreciated thatwhere the basis set of OLED materials are appropriately chosen,maintaining the proper color basis set (for example Red, Green, andBlue) integrated photon flux will also maintain the color balance of thepixel. The display device 201 also includes sensors and sensors coupledwith capacitors to form novel photon flux integrators (in one embodimentsensor S1 coupled with capacitor C2) associated with each individualpixels for measuring a characteristic of the perception of luminancebased on an integrated photon flux over an integration period T_(PFI).The sensors 203 generate a sensor output signal 207 (and in oneembodiment a plurality of sensors generate a plurality of sensorsignals) that is (are) communicated to the display drive and controlelectronics 204 and used by the drive and control electronics 204 tomodify the pixel drive and control signal(s) 206 as necessary to achieveand maintain individual pixel photon flux levels and achieve uniformityperformance between and among the plurality of pixels in the display. Inone embodiment there is a sensor 203 associated with and located withinor adjacent to each pixel 202 so that the pixel integrated photon fluxand uniformity is achieved on a pixel-by-pixel basis rather thanglobally for the entire display.

The inventive device, system, and method also advantageously provide forthe measurement of the integrated photon flux for each pixel separatelyand such measurement is not limited to the measurement of a row ofpixels, a column of pixels, or any other set of collection of pixelstogether. Embodiments of the invention also provide for separate pixelsenor output signals so that it is not necessary to sense or measure acurrent, voltage, or other indication of photon flux, integrated photonflux, or luminance serially over a common sensor line.

This pixel-by-pixel approach is particularly advantageous as it permitsadjustments and corrections to each and every pixel to account foroperational history differences of each and every pixel so that in spiteof these historical operational differences, the same or any desiredpixel integrated photon flux can be achieved. For OLED display pixels orother display types where the integrated photon flux and otheroperational parameters at any point in time are highly dependent on pastoperational history at the individual pixel level, this solves thedisplay aging, display and pixel “burn-in” problem, and other operatingor age related problems.

Another embodiment of the invention incorporates at least some of thefeatures of the FIG. 3 embodiment as well as additional features. Inthis embodiment, the image data 205 is received from or generated by ananalog image source 208 that provides an analog signal, such as an RGBcomposite signal, separate component red (R), green (G), and blue (B)signals, a monochrome or black/white signal, or any other source or typeof graphical, text, symbolic, image, picture, or other data. This datamay be dynamic (that is changing over time) or static. Examples of suchimage data are television (TV) analog or digital signals, computerdisplay signals (such as half-VGA, VGA, super-VGA, any of the digitaldisplay interfaces, and the like), cellular or mobile telephone displaydata, watches, appliances, automotive electronics display data (such asfor example automotive instruments, navigation, and entertainment),aircraft avionics and in-flight entertainment, fixed and portable gamingdevices, billboards and other large displays, and any other type ofdisplay and data.

When the image data is in the form of sequential or serial frames orsegments of analog data (such as a conventional television signal), thedata signal 205 (See FIG. 3) is processed by serial-to-parallel (S/P)and analog-to-digital (A/D) processor circuitry or logic 209 to generatedigital red (R_(D)). digital green (G_(D)), and digital blue (B_(D))signals. It may be appreciated that monochrome or black/white signalsmay be achieved for a color display by providing the same integratedphoton flux levels from adjacent R, G, and B emitters or pixels(sometimes referred to as RGB subpixels). Alternatively, where only amonochrome display is provided, then only a single pixel signalrepresenting the display image is required rather than three (RGB)signals. Furthermore, where the image data is already in parallel and/ordigital form, either or both of the serial-to-parallel andanalog-to-digital conversion or processing may be eliminated. For easeof description it will be assumed for purposes of this description thatthe display is a color display and uses Red 210-1, Green 210-2, and Blue210-3 signals which will conveniently be referred to as the digitalimage input data 210; however, it will also be clear that the inventionapplies to monochrome displays with only one digital input data signal.The description will also user the more usual nomenclature of R, G, B,or simply RGB signals to describe the three signals or data setstypically associated with a color display or image. Whether such signalsor data are analog or digital will be apparent from the context of thedescription. The RGB nomenclature will also stand for any set of colordyes, phosphors, filters, or materials the form a color or colors, orother basis sets (independent on number of color basis element) that maybe used to produce a true, false, or pseudo-color display.

Normal display operation is carried out by the blocks in FIG. 4 namely,Analog Image Data 208, Image A/D converter 209, Gray level logic Z103(modified to accept an inventive input), Display Controller Z104, ColumnDrivers 238, Row Select 240, and the Active matrix Emissive display 292,293, 294. Optionally the Analog Image Data Block 208 and the Image A/Dconverter 209 may be replaced by the Digital Image Data Block 208 a. Ineither case, digital image data is fed into the Gray Level Logic block.

A top level description of each of the blocks in the FIG. 4 embodimentis provided, followed by additional detail where warranted. The Displaycontroller Z104 controls all timing signals, converts image voltage datainto display voltage data. Column drivers 238 down loads or otherwisecommunicates display voltages to the rows. Row Select logic 240 enablesthe rows one at a time to receive data from column drivers. Sample andHold block Z101 samples and holds the sensor data from each row as it isaddressed by the row select. Analog-to-Digital (A/D) converter 270 isresponsible for converting the analog data at the Sample and Hold blockZ101 to digital data. Multiplexer (MUX) 270 a coverts the parallel dataat the A/D converter into a serial data stream. Calibration Memory 250stores the original sense data that was taken when the display was firstmanufactured, by pixel and by gray level. Comparator 260 is responsiblefor performing a comparison (such as a magnitude or differencearithmetic comparison) between the pixel emission data and thecalibrated data. The Digital or Pixel Deviation Memory Z102 stores thedeviation from calibration for each pixel and gray level. Gray LevelLogic Block Z103 may be responsible for (i) determining a gray levelstrategy (simple voltage, spatial and/or temporal dithering or the likefor achieving a desired luminance), (ii) for determining when to sendcorrections to display driver controller, (iii) and for determining oridentifying how to correct the display driver controller using the datastored in the digital deviation memory. Analog Image Data block 208sources image data in an analog format when the data is provided in thisform (becoming obsolete). Digital Image Data 208 a sources image data ina digital format (more and more prevalent today) Image A/D Converter 209converts the analog image data to digital.

The Gray Level Logic block Z103 converts the digital image data into aform which can be used by the Active-Matrix emissive display to recreatean image faithfully corresponding to the image data. Although functionalblocks having some of the features of the Gray Level Logic block of theinvention are known in the art, they are not the same as used in theinventive system and method, at least in part because the inventive GrayLevel Logic block Z103 includes an input for receiving values from PixelDeviation Memory Z102 (described in greater detail below) and structuraland methodological means for using both the output of image A/Dconverter 209 and outputs from the pixel deviation memory Z102 toprovide novel inputs to Display controller Z104.

The Gray Level Pixel Logic function block Z103 may be any circuit,logic, digital function (optionally including software and/or firmware)or any other hardware, software, or hybrid hardware/software means thatconverts the digital gray level determined by the inputted image data toa voltage calculated to cause the pixel specified to emit luminance atthe same gray level as required by the image.

It is understood in light of the description provided here that thereare many ways to reformat the image data to be able to produce a displayimage with proper gray levels and colors. For example, the Gray LevelLogic block may include a gamma function which transforms image voltagedata into display voltage data that will produce the proper luminancechanges from one gray level to another. Another function that mayoptionally be included in the Gray Level Logic block would be a systemto effect gray levels by using temporal dithering; that is, by dividingeach frame into two or more sub-frames. Operating on x number of graylevels using just one sub-frame (the other always remaining in the darkstate) would allow the doubling of the levels by using both sub-framesin the on state. The Gray Level Logic block may also arrange to usespatial dithering for gray levels. This means that each pixel would havean array of sub-pixels, which would be turned on or off depending on thegray level. Some limited forms of this approach are already used colordisplays in order to use the three primary colors to reproduce all thecolors in the visible spectrum. The Gray Level Logic block could alsouse a combination of temporal and spatial dithering to accomplish thegray level function. The data that emerges from the Gray Level Logicblock is sent to the Display Controller Block. The Display Controllerblock literally runs the display. It provides all the timing signalsthat control sending the display voltage data to the column drivers, andit provides the timing of the row selection driver so that the properrow is selected for the particular line of data being down loaded to thedisplay from the column drivers. The Gray Level Logic block Z103determines what voltages will be down loaded, and the Display controllerdetermines when the voltages will be down loaded. The Column Driversreceive the digital voltage level for the first row of the frame,converts the digital data to analog data, and downloads the data to thefirst row of pixels which has meanwhile been selected by the row selectdriver under the command of the Display Controller. Since this is anactive-matrix display the data voltages are stored on a storagecapacitor and to the gate of the current controlling TFT, thus turningon the OLED in the pixel. The display controller then sends the next rowof data and selects the next row of the display and so on until all rowsin the frame have been activated. There is then a retrace to the firstrow and the next frame commences to be down loaded by the DisplayController. These aspects of display operation are known in the art andno further detail provided here.

The functional blocks and structure relating separately and incombination to aspects of the invention are the Sample and Hold Z101,the A/D converter 270, the multiplexer or MUX 270 a, the Comparator 260,the Calibration Memory (Cal Mem) 250, and the Pixel Deviation MemoryZ102. The Gray Level Logic block Z103 is also a modified version ofconventional gray level logic because it includes structural and methodcomponents that permit it to accept and utilize the output of the pixeldeviation memory which are themselves based on the results of comparator260. In this embodiment of the invention there are two memory blocks theCalibration Memory 250 and the Pixel Deviation Memory Z102. In otherembodiment there may be more memory block or less. In the interest oflower cost, the less memory the better. It is, however, easier tounderstand the principles of invention by referring to the two memoryblock in this embodiment. Other embodiments my readily use a singlememory. In the invention a photo-sensor system called a photon fluxintegrator has been added to the pixel. When the pixel is activated bythe data sent by the column drivers light is emitted in the form of aphoton flux from the OLED. A portion of that photon flux is interceptedby the photo-sensitive material in the photon flux integrator, convertedto electrons and collected by the capacitor in the photon fluxintegrator. The collection of photo-electrons continues for the fullduration of the frame (at a 60 Hz frame rate this is a time duration of16.7 ms). On the next frame, the charge or voltage on the photon fluxintegrator capacitor is read by the Sample and Hold Function out sidethe display area. In one embodiment the voltage on the capacitor is readand in another embodiment the charge on the capacitor is read. Thecharge and voltage on the capacitor is proportional and is someembodiments it is directly proportional to the luminance of the pixelduring the frame time.

While there are many ways to read voltage and charge known in the art,FIG. 5 and FIG. 6 give examples of two embodiments. These circuits andvariations of them are described relative to embodiments in FIG. 7 andFIG. 8. It will be appreciated that the circuits and methods for readingvoltage and charge (or current) are known in the art and that thecircuits and methods described here may be applied to a variety ofdifferent pixel circuits and structures, including to different pixelemitter circuits, pixel sensor circuits, and/or pixel photon fluxintegrator circuits.

The FIG. 5 embodiment is a voltage sensing circuit. Line L1 suppliesvoltage to both power transistor T2 and sensor S1. The dark resistanceof sensor S1 is extremely high and sensor capacitor C2 receives verylittle charge through S1 when the pixel is off. During the frame timewhen OLED or other emissive device or diode D1 (such as an OLED) is inthe on state and a photo flux is received by S1 the conductivity of S1significantly increases and allows charge to flow into sensor capacitorC2 causing a voltage to appear across C2 with respect to ground. (Notethat the combination of Sensor S1 and sensor capacitor C2 in the contextof the rest of the circuit are operative to form a photon fluxintegrator device.) This voltage is proportional to the photon fluxlevel emitted by D1. In order to read the voltage on C2, sensor TFTtransistor T3 is turned on by applying a voltage to line L2 (this occurswhen the row is enabled). The voltage of sensor capacitor C2 issubsequently applied to the plus terminal of an operational amplifier(op amp) OA1 or equivalent amplifier circuit. The negative terminal ofthe operational amplifier OA1 is coupled to a reference node such asground G2. This voltage is amplified by the ratio of resistor R2 (in thevoltage sensing amplifier to the line resistance of L4 which is coupledto the positive input of operational amplifier OA1. For example, if theline resistance of line L4 is 3K ohms and the resistance of resistor R2is 3 Mohms, the voltage on capacitor C2 is amplified by 30 dB (1000times), which voltage appears at node P4. The amplified voltage is sentto a sample and hold circuit for further processing.

Another embodiment is shown by FIG. 6. In this embodiment, when avoltage, for example, 10 volts, is applied to the plus terminal ofcharge amplifier CA1, line L4 quickly also ramps up to 10 volts. Aresistor R1 is coupled between the negative input terminal of the chargeamplifier and its output at note P3, and capacitor C3 is connected inparallel across resistor R3. The voltage appearing at node P3 is anoffset voltage determined by the characteristics of charge amplifier CA1and any leakage current on L4. This leakage current typically may arisefrom the fact that in a multi-row display each row will have atransistor T3 attached to line L4 and although the T3s is every rowexcept the row that is enabled will be in the off state there still isan off state current leakage associated with each T3. Capacitor C2 ischarged up to the voltage on the plus terminal of CA1 when T3 is turnedon. Any charge flowing into C2 reduces by the same amount the chargeacross C3 and the voltage rises on node P3. Resistor R1 may usually be alarge resistance that allows the reduced charge on C3 to be restored forthe next reading. In practice a reading of P3 is advantageously madeprior to turning on transistor T3 in order to measure the offsetvoltage. Then another reading is made after T3 is turned on and thefirst reading is subtracted from the second reading to give a value forthe amount of charge that flowed into C2. Therefore, as in theembodiment of the circuit of FIG. 5, the photon flux from D1 causescharge to move from C2 to ground during the frame duration. When line L2is again selected for the next frame, the charge on C2 is read by thecharge amp circuit.

The column drive unit 238 works in conjunction with line buffer 236 androw select unit 240 too sequentially select and sends pixel signals toeach subsequent row of the display. The operation of column drive unit238 and row select unit 240 are generally known in the art and notdescribed in further detail here.

A sensor 294 is positioned or disposed within or adjacent to pixel 292so that it can receive at least a portion all of the light, photons, orother radiation that may emanate from pixel 292 when the pixel is drivenby the column drive circuitry at a level what causes it to emanate. Thesensor 294 may also be responsive to ambient light or radiation levels.Sensor 294 may be any type of sensor that undergoes a measurable changein physical or electrical characteristic in response to different levelsof the incident light or radiation.

Sensor 294 therefore generates an electrical signal, in the form of aphoto current that is a measure of, or otherwise indicative of, theincident photon flux on the sensor during the period of the frame timeof the measurement. In one embodiment of the invention, the sensormeasures the integrated photon flux over a defined period time. In atleast one embodiment of the invention, the defined period of time is theframe period. It is noted that most displays operate at a frame rate ofat least 60 Hz so that the content (such as a image) displayed does notappear to flicker to a human observer. A frame rate of 60 Hz correspondsto a frame time or period of substantially 16.7 ms. Other displaysoperate at higher frequencies to further reduce the possible flickering.A frame rate of 100 Hz corresponds to a frame time or period ofsubstantially 10 ms.

The invention is not limited to any particular frame rate, and isapplicable to non-interlaced and interlaced display types. Furthermore,while much of the description indicated that the photo flux isintegrated for a period of exactly or substantially the display frametime, there is no reason why the photon flux integration need extend forthe full frame time so long as the time is long enough to provide anintegrated photon flux of sufficient magnitude in absolute terms andrelative to the noise, and so the positive and negative contributions torandom noise cancel within required margins. It is anticipated thatphoton flux integration times on the order of between at leastone-quarter of a frame time and one frame time may readily be used, andthat photon flux integration times as short as about one-tenth of aframe time (e.g. 1.67 ms) may also be used as this still provides a gainof 333 times as compared to the typical 5 μs instantaneous time formeasurement in the prior-art as explained in the previous example. Evena photon flux integration time of between one one-hundredth andone-tenth of a frame time may provide satisfactory performance.Typically the integration time will be one frame time so that a singleset of control and timing signals may be used for the pixel writeoperations and integrated photon flux sensor read operations. It isanticipated that even time frames as short as the row address times maybe practical with the use of noise canceling circuitry.

It is noted that most displays operate at a frame rate of at least 60 Hzso that the content (such as a image) displayed does not appear toflicker to a human observer. A frame rate of 60 Hz corresponds to aframe time or period of substantially 16.7 ms. Other displays operate athigher frequencies to further reduce the possible flickering. A framerate of 100 Hz corresponds to a frame time or period of substantially 10ms. The invention is not limited to any particular frame rate, and isapplicable to non-interlaced and interlaced display types.

If photon flux is measured in photons/second/meter-squared, then thesensor is integrating or counting the total number of photonsintercepted over the sensor area during that time period so that sensoris acting as a photon counter and not as an instantaneous detector ofphotons, electrons, or other energy or particle. The integration overtime permits the acquisition of a single magnitude sufficient toovercome instantaneous noise that may be present and of a signal that isrelatively stable from frame to frame assuming that there are no changesin the display pixels or the electronics that drive the display pixels.

It will be appreciated that each pixel (really each subpixel whenimplemented in a tri-color RGB color display) within each display rowhas an associated separate sensor 294, and that each sensor 294generates and communicates a sensor output signal 207 to off-displayglass electronics. In one embodiment this sensor output signal is avoltage (Vs), but in other embodiments the sensor output signal is acurrent (Is). Additional signal processing structures or circuits may beprovided either within the pixels or subpixels, display, or inoff-display glass processing circuitry to convert from one signal typeto another and/or to derive a different signal from the raw sensorsignal. In order to simplify the discussion, this description is limitedto the manner in which the sensor signal 207 from a single particularsensor is processed through the drive and control electronics 204 toachieve the desired operation and display uniformity. In reality eachpixel (and sub-pixel) has a sensor that generates and communicates asensor output signal 207 to off-display electronics so that apix-by-pixel (and subpixel-by-subpixel) measurement and feedback basedcorrection can be made. In a separate portion of this description, thecalibration and operational procedures will described the manner inwhich pixel sensor data is used to correct display nonuniformity.

Sensor output signals 207 (one for each column in the display) aresimultaneously captured by Sample and Hold Z101, processed byanalog-to-digital (A/D) converter 270 and MUX 270 a to convert thenormally parallel analog signals 207 into serial digital signals orvalue Vs 276. This digital sensor signal 276 is received by a signalcomparison unit 260 that is responsible for comparing the measured pixelintegrated photon flux (as indicated by the sensor output signal 276)with a reference pixel integrated photon flux value 251 that correspondsto the expected pixel gray level stored in calibration memory 250. Itwill be appreciated that signal levels may be scaled or otherwiseprocessed so that the comparison unit 260 compares signals having thesame scale or range so that precise and accurate differences can becomputed. The difference between the reference value and the sensedvalue for that particular pixel is referred to as the difference ordelta gray scale Δ_(GS) amount and is sent to Pixel Deviation MemoryZ102.

The reference voltage stored in calibration memory 250 may be generatedin any number of different ways. In one embodiment the values placed incalibration memory 250 are generated at the manufacturing point wherethe active-matrix back plane has been completed before the OLEDmaterials are deployed over the back plane. At this point theactive-matrix is fully exposed to ambient luminance. Therefore, thedisplay may be sequentially exposed to calibrated gray levels and eachsensor scanned as though in normal operation with the measured sensorvalues being electronically stored and later introduced into calibrationmemory 250. Another embodiment uses a procedure in which displaymanufacture is completed, which includes adjusting the Gray Level Logicblock Z103 to produce the desired color mixing and luminance uniformityusing practices well known in the industry. When the display is firstbooted up or turned on it may enter a calibration mode where it isassumed that the first sensor values are correct since the display hasno aging history. These first values are stored in the calibrationmemory and subsequently used to maintain the initial condition of thedisplay.

The Pixel Deviation Memory Z102 contains the status of all pixels withreference to the initial conditions, or to initial calibration inmanufacturing. It is the purpose of the Gray Level logic functionalblock Z103 to produce the correct digital voltages that will faithfullyreproduce the image data on the display. Procedures for accomplishingthis are well known in the display industry and therefore not describedin further detail here.

In embodiments of the present invention the decisions made by the Graylevel logic function are modified by the data stored in the PixelDeviation Memory. In one embodiment, for example, if the data in thePixel Deviation memory indicates that pixel has degraded by two graylevels, then the Gray level Logic function adds two levels of gray scaleto the normal digital voltage level determined for the image data.Another embodiment would be to subtract two levels of gray from all theother pixels and thus maintain color balance, but decrease the dynamicrange of the display. Another embodiment use an approach wherein the ontime of the degraded pixel is increased in order to increase itsperceived luminance by two gray levels. Other embodiments involvespatial and/or temporal dithering using techniques will known in theindustry.

Embodiments of the invention provide for performing the calibration atany time either automatically according to some rule, policy orschedule, or manually by the user. Automatic calibration is preferred.Two particular schemes are to perform the calibration every frame, atsome integral number of frames interval where that interval can be anynumber, a power-on, at power-down, at some elapsed time interval (e.g.every 1 hour) or according to any other scheme. It will be appreciatedthat the user is not aware that the calibration is occurring and thereis no or substantially no loss or overhead associated with thecalibration once the structures for performing the calibration are inplace. Operations such as additional write operations to memory and/oradditional switching or logic operations represent the only additionalactivity, but these are inconsequential compared to the other operationsthat occur.

These and the other circuits described herein may be implemented asintegrated circuits either on the same substrate as the display (e.g.the display glass) or on separate substrates off the display. In generalthe control system elements may advantageously be provided off of thedisplay substrate. In particular embodiments of the inventive controlsystem and circuits provide the sample and hold circuits Z101,analog-to-digital converter circuits 270, multiplexer 270 a, comparatorcircuits 260, calibration memory 250 and pixel deviation logic Z102 aand pixel deviation memory Z102 b. The display controller Z104, graylevel logic Z103, and image A/D converter 209 may also advantageously beimplemented as one or more integrated circuits off of the displaysubstrate. Embodiments of the pixel circuits described in detailhereinafter are implemented as structures for each pixel on the displayglass or substrate.

2. Exemplary Pixel Device Structures and Circuits

One aspect of the invention provides a conversion from a high impedanceto a low impedance. The conversion from high impedance to low impedanceoccurs at least in part because of the structure, configuration, and/oroperation of the sensor capacitor. The sensor operation of charging ordischarging the sensor capacitor C2 is a high impedance operation sincethe sensor has gig-ohms of resistance. During this charging ordischarging time, the sensor line is isolated from the high impedance bysensor transistor T3. During the read time sensor transistor T3 isopened connecting the sensor capacitor C2 (which had been isolated fromsensor line L4) to the sensor line L4.

Impedance between the sensor capacitor C2 and the sensor line L4 is onlythe resistance of the sensor line, which would normally be only about 3Kohms for typical implementations. The impedance difference is thereforeon the order of one million to one (10⁶:1). Interference from noiseresults in nano-amps of current flow which in a gig ohm. impedancesystem amounts to noise that is on the order of volts, but in a kilo-ohmimpedance system amounts to micro-volts. Since it is the long length ofthe sensor line L4 in a typical display implementation that picks up thenoise interference, a measurement should preferably not be made when thesensor line is connected to a high impedance system. When the sensor S1is isolated by sensor TFT T3 any noise affecting the sensor S1 has to bepicked up by the extremely short lines of the pixel circuitry;therefore, very little if any noise affects the charging or dischargingof the sensor capacitor. These switching and impedance characteristicscontribute to the successful operation of the pixel and sensor circuits.

Two exemplary pixel with sensor circuits are now described that may beused with the inventive display, display control system and method, andsensor readout circuits and methods. Although particular pixel emitter,sensor, and circuit topologies are described relative to these twoembodiments, it will be appreciated that the invention is not limited toonly these particular circuits or device structures and that variationsin the design and the particular electrical circuit devices may bemodified, such as by changing the types of control devices to be otherthan particular transistors, TFT, diodes, or the like and substitutingany two-terminal or three-terminal control or switching means. While thetransistors are indicated as being TFT type transistors, the inventionis not limited to only TFT type transistors. Furthermore, otheralterations to pixel circuit topology, such as by adding additionalcircuitry may be made without departing from the spirit and scope of theinvention. The type of emissive device may also be modified to be otherthan an OLED emitter and for example any active emitter may be usedincluding but not limited to inorganic photon emitting devices orstructures; and the characteristics of the sensor may be modified sothat in addition to photoresistive or photoconductive devices, anysensor device that undergoes a change in response to incident photonflux may be substituted

One of the advantages of both of the circuits described relative to theembodiments in FIG. 7 and FIG. 8 are that they provide a high-impedanceto low-impedance conversion system for an active matrix emissionfeedback stabilized flat panel display, such as an OLED display. Thecircuits of FIG. 7 and FIG. 8 provide this by isolating the off displayglass or substrate circuitry (such as voltage comparator amplifier VC1and switching transistor TFT T4) from the high impedance of sensor S1 inthe pixel during the photon flux integration operation, which occursduring the frame time. The design of the circuits prevents noise onsensor line L4 that would result if sensor line L4 was connected to ahigh impedance source.

In this regard, it is well known that a conducting line connected to ahigh impedance will pick up electromagnetic interference from theenvironment. This is easily demonstrated by observing the behavior of avolt meter with the plus and minus leads open in the air. The voltagewill continually range from plus a few volts to minus a few volts due toradio and TV interference. Since S1 has a resistance in the gig-ohmrange and higher, it acts like an open circuit to sensor line L4 if L4is connected directly to sensor S1 without benefit of sensor capacitorC2. During the photon flux integration time sensor TFT T3 is turned off.While power supply line L1 is not isolated from sensor S1 in this pixelcircuit configuration, noise on power supply line L1 does not affect theoperation of the pixel or the display since power TFT T2 is operating inthe saturation mode and therefore changes of voltage (even on the orderof volts) across power TFT T2 due to noise does not change the currentthrough T2, and therefore the emission of photons from pixel diodeemitters D1 for all pixels in the display remains stable.

Furthermore, any noise picked up by power supply line L1 fluctuatesaround zero volts (that is on average it has substantially equivalentpositive and negative fluctuations about zero volts) during the frametime when sensor capacitor C2 is charging through sensor S1; therefore,the noise cancels out and the voltage on sensor capacitor C2 after theframe time is complete is due only to the discharge rate of sensor S1when photons are intercepted. During the row address time when thevoltage on line selection voltage line L2 goes high and turns on driveTFT T1 and sensor TFT T3, the voltage on sensor capacitor C2 is read bythe voltage comparison amplifier VC1 at its sensor input on P1. Thissensor input at P1 is compared with a reference voltage at P2 on itsother input to generate a difference or error voltage at output P3.Noise does not interfere during the reading of the voltage present onsensor capacitor C2, because the current induced by noise is in thenanoampere range and at most may cause slight changes in the charge oncapacitor C2, but since virtually no current goes though the highimpedance no voltage results from the low level of noise interference.

One of the primary differences between the circuits of the embodimentsof FIG. 7 and FIG. 8, is that in the FIG. 7 circuit embodiment, thevoltage on sensor capacitor C2 at the beginning of the frame is zerovolts and provided by turning on grounding TFT T4 at the end of the readtime during the row address time. The voltage on the other side ofsensor capacitor C2 is at the line L1 voltage which is the supplyvoltage to power transistor T2, which may for example be at +10 volts.As sensor S1 in combination with sensor capacitor C2 integrates thephoton flux from OLED D1 over the frame time, the voltage at the pointP5 between C2 and T3 rises toward the supply voltage on L1 (e.g. toward+10 volts). The more photons received by sensor S1 and integrated by thecombination of sensor S1 and sensor capacitor C2, then the closer thevoltage between sensor capacitor C2 and sensor transistor T3 comes tothe supply voltage on line L1. While this circuit has many advantagesover conventional circuits and methods, a possible drawback of thisparticular embodiment of the circuit in an actual implementation is thatthe supply voltage on line L2 may possibly fluctuate a small amount dueto the number of pixels and the level of OLED emission from each pixelbeing supplied by L1. Since this can be any combination of pixels andemission levels the voltage reading on sensor capacitor C2 maytheoretically have some slight ambiguity but this ambiguity maygenerally be small and performance still an improvement overconventional circuits and methods.

The circuit 380 described by FIG. 8 on the other hand is referenced toground and to the voltage of Vcap 355 that is fed or communicated tosensor capacitor C2 327 through the sensor TFT T3 330 and TFT T4 340transistors during the address time.

Although the two circuits have a somewhat different structure andoperation, they have certain features in common. In each of thecircuits, an emissive device (such as an OLED diode) coupled to groundis driven by a controlled current source (such as a TFT transistor T2).The pixel data value in the form of a voltage is applied to the controlterminal (TFT gate) so that the pixel emission (number of photons) isrelated to its intended integrated photon flux. Recall that a sensor S1324 and a capacitor C2 327 are coupled as a photon flux integratordevice 339 (along with supporting circuitry) with the pixel emissiveelement (OLED diode) so that a representative and measurable number ofthe photons emitted from the emitter are incident on the sensor and thecombination of the sensor and capacitor generates a photon count. Thesensor S1 and capacitor C2 combination integrates or counts the totalnumber of photons it has collected during a defined period (in oneembodiment the display frame time of 16.7 milliseconds). This integratedphoton flux is a useful measure because it provides greaterrepeatability and immunity from noise than any instantaneous measure,provides a larger signal amplitude, and the integrated nature of thephoton flux may likely be more representative of the integrated photonflux perceived by a human observer owing to the relatively slow responseand latency of the human visual system.

A reference integrated photon flux has been established, and the sensorsignal is then communicated to the control system and used with thereference to adjust the data signal that is applied to the controldevice during the next calibration period (such as the next frame) sothat the actual pixel integrated photon flux (effectively photonsemitted by the OLED diode or other emitter) matches the desiredintegrated photon flux (number of photons identified duringcalibration).

Having now described some of common aspects of the pixel circuitstructure and operation, attention is now directed to a more detaileddescription of the two embodiments illustrated in FIG. 7 and FIG. 8.

An embodiment of an active matrix display pixel with emitter, sensor,photon flux integration, and control components is now describedrelative to FIG. 7. A pixel diode drive transistor T1 310 is coupled toa image voltage line L3 301 at its drain (D_(T1)) terminal 311, to afirst terminal 315 of storage capacitor C1 314 and to the gate terminal(G_(T2)) 323 of a power control transistor TFT T2 320 at its source(S_(T1)) terminal 312, and to a line selection voltage line L2 302 atits gate (G_(T1)) or control terminal 313. Power TFT transistor T2 320is coupled to power supply voltage line L1 301 at its drain terminal321, and this drain terminal is also coupled to a first terminal 325 ofsensor S1 324 and to a first terminal 328 of sensor capacitor C2 327 ata common node. A second terminal 316 of storage capacitor C1 324 iscoupled to the source terminal 322 of power TFT T2 320 and to the inputterminal 337 of emitter (OLED diode) 336. The output terminal 337 ofOLED emitter 336 is coupled to ground 305. A second terminal 326 ofsensor S1 324 is coupled to the second terminal 329 of sensor capacitorC2 327. A calibration read voltage (Vcal) is measured or read at node P5334 defined by the connection of sensor S1 output at 326 and the sensorcapacitor terminal 329 as described hereinafter. This node P5 is alsocoupled to the source terminal 331 of sensor TFT T3 330. Sensor TFT T3330 is also coupled at its source terminal 332 to sensor line L4 304which provides an input signal at an input port P1 351 of voltagecomparator VC1 350. Voltage comparator 350 receives a reference voltageat a second input port 352 and generates a difference or error signal P3353 computed as the difference between the P1 351 and P2 352 inputs. Inthis embodiment, the sensor output that is applied as an input to thevoltage comparator VC1 350 is also applied at a common node 351 as thedrain terminal 341 input of grounding TFT T4 340. The source terminal342 of TFT T4 340 is coupled to ground 306, and receives a controlsignal 344 at its gate terminal 343. These transistors provide switchingto connect pixel elements at times and to isolate other pixel elementsat the same or different times so that tight management, control, and ormeasurement of small voltages, currents, charges, and/or photon countsmay be precisely and accurately accomplished. Note that the sense ofsource and drain terminals of the TFT may be reversed depending upon then- or p-type of material used for the TFT transistors.

While certain elements of the circuit described cooperate and contributeto operation of the pixel emitter, the pixel photon flux integrator, andthe measurement and calibration operation, some approximate categoriesmay be developed to assist the reader in understanding aspects of theinvention; however, these categorizations are should not be applied tolimit the scope of the invention as elements of the circuit describedcontribute to more than one category at some times and not at all atother times as described in detail in this specification. With this inmind, drive TFT T1, storage capacitor C1, power control TFT T2 and diodeD1 may contribute primarily to operation of the OLED diode emitter;sensor S1, sensor capacitor C2, and sensor TFT T3 contribute primarilyto the operation of determining or generating an integrated photon fluxmeasurement; and voltage comparator VC1 and grounding TFT T4 in thisembodiment contribute primarily to reading the integrated photon fluxmeasurement and determining a difference between that measurement and areference so that a correction may be applied to adjust the pixelemitter luminance as indicated by the measured integrated photon flux.

Having described the general topology and connectivity of the circuitelements in FIG. 7, attention is now focused on its operation so thatadditional aspects and advantages of the invention will be betterappreciated. A power source voltage (V_(PS)) typically in the range of10 to 15 volts is applied to line L1 301, which serves as the powersource for both OLED D1 336 and the charging source for sensor capacitorC2 327. The invention is not limited to any particular range and higherand lower voltages may be used consistent with device characteristics.At the same time, a line selection voltage (V_(LS)) is applied to lineL2 302 causing data drive TFT T1 301 to turn on. Also at the same timean image voltage (V_(IM)) representing the image to be displayed andreferred to as the image voltage is applied to line L3 303, and due tothe fact that data drive TFT T1 301 is turned on (or conducting), thisimage voltage (V_(IM)) is delivered by TFT T1 to the gate G_(T2) 323 ofpower control TFT T2 320 and storage capacitor C1 314. This causes adevice current (I_(D1)) to be delivered by TFT T2 320 to OLED D1 336 anda specific light emission level is emitted from OLED D1 336 that iscalculated to be the proper light emission (E_(CALC)) required by theimage. When the display is new and freshly adjusted by the manufacturerthe image voltages will produce the correct pixel/OLED emission values.In one embodiment, sensor S1 324 is physically located in contact withthe semiconductor anode side of the OLED D1 336 for optimum opticalcoupling so that sensor S1 collects or intercepts at least a portion ofthe light emitted by OLED during its emission, and preferably as much ofthe emitted photons as possible so as to improve integrated photon countand signal strength. In terms of luminance, in this embodiment sensor S1receives the same or substantially the same luminance as the OLED pixelemits, because the flux density striking the pixel (the sensor portionof the pixel) is the same as the flux density emitted by the pixel (theemitter portion of the pixel) as a whole because the portions arepreferably (but not necessarily) in contact. (Other embodiments providethe sensor S1 to be physically located near the OLED so that it collectsor intercepts enough light to provide useful sensor signals but not incontact with the anode side of the OLED D1.) In one embodiment, thesensor S1 is a photoresistive (or photoconductive) sensor in which theresistance decreases (or conductivity increases) with increasing photonflux density emitted by the OLED emitter.

During the frame duration (T_(FR)), which at 60 frames per second (fps)is 16.7 ms, the light emitted from OLED D1 336 impinges on sensor S1 324and causes a resistance (R_(S1)) 347 component of the sensor S1 324 todecrease in proportion to the intensity of the light (photon) emission.During the display frame time, sensor capacitor C2 327 is beingdischarged through sensor S1 324. The frame duration and the averageresistance (R_(ave)) 348 of sensor S1 during the frame time determinethe amount of charge discharged by sensor capacitor C2. The amount ofcharge discharged by sensor capacitor C2 is an important parameterbecause it controls or determines the voltage (V_(CAL)) on the node P5connected between sensor capacitor C2 and sensor TFT T3. This readcalibration voltage will be the read value sent to the circuit or otherlogic that determines the correction that is used to calibrate andmaintain the uniformity and color balance of the display during normaloperation. (Different embodiments of the invention provide differentread circuits which are described elsewhere in this specification.) Itis important to note that the higher the voltage measured at the node P5between sensor capacitor C2 and sensor TFT T3, the greater amount ofphoton flux (pixel luminance) that was detected or intercepted by sensorS1. This happens because the lower the resistance of S1, the closer (orthe smaller the difference) the voltage at the node P5 between sensorcapacitor C2 and sensor TFT T3 comes to the supply voltage on L1.

With reference to FIG. 8, there is illustrated a second embodiment ofthe present invention. Like numbered elements in this specification havethe same or similar operation unless such differences are described.There are many similarities between the two circuits and the entiretopology and connectivity of elements is not repeated here. In thisembodiment sensor capacitor C2 327 is first charged to a predeterminedvoltage as it was in the first embodiment of FIG. 7 using the powerline, but in this embodiment sensor capacitor C2 327 is charged throughthe sensor line by TFT T4 340 and a capacitor charging voltage source(Vcap) 355, such as for example to +10 volts (or to any other voltagevalue). (Recall that the FIG. 7 embodiment does not utilize a capacitorcharging voltage Vcap in this manner and note that the TFT T4 transistoris operable to interact between the P1 input of the voltage comparator350 and Vcap 355 rather than between the P1 input and ground 306.)

During the frame time (for example, a frame time of about 16.7 ms for a60 frame/sec (fps) system), light or photons from the OLED D1 causes theresistance of sensor S1 324 to decrease and accelerate the discharge ofsensor capacitor C2 327 to ground. As compared with the FIG. 7embodiment, in this FIG. 8 embodiment the voltage on sensor capacitor C2336 moves towards the ground voltage at G1 305 (or other voltage)instead of moving towards the positive supply voltage as in the FIG. 7embodiment. Therefore, the greater the photon flux emission from OLEDD1, the lower the resistance of sensor S1, the greater the currentduring the frame time discharge, and the lower the voltage remaining onsensor capacitor C2 when sensor capacitor C2 is measured during the readtime. This FIG. 8 embodiment therefore has advantages over the FIG. 7embodiment, because the charge voltage may be better controlled on thesensor line L4 than it is on the supply voltage line L1, but bothembodiments are useful and have significant advantages over conventionalcircuits and methods. In general for an actual implementation, thevoltage on supply voltage line L1 varies according to the amount ofcurrent being delivered by line L1 and the row being measured. For manydisplay architectures, the higher the row number the further away therow will be from the line L1 power supply and more current timesresistance (I*R) voltage drop in the line to that row. By comparison,because the sensor line L4 in this embodiment only delivers current whena reading or measurement is being made, or when sensor capacitor C2 isbeing re-charged, the voltage is highly stable and not subject topossible variations as the supply voltage line may be so subject.

These and the other circuits described herein may be implemented asintegrated circuits either on the same substrate as the display (e.g.the display glass) or on separate substrates off the display.

3. Embodiment of Calibration of the Sensors and Circuit

The sensors may be calibrated during manufacturing before the display iscompleted (pre-manufacture calibration) or after manufacturing has beencomplete (or at selected stages in between these two times). The firstembodiment of calibration is the calibration during manufacturing. FIG.9 is an illustration showing an embodiment of the calibration flow chartfor pre-manufacturing calibration. The point of calibration is after theactive-matrix and sensor circuitry has been completed, but before theOLED structure has been deposited on the active-matrix back plane. Atthis point the completed active matrix back plane is inserted into atest fixture that connects all the display inputs except the L1 supplyvoltage to a display control board which drives the active-matrixbackplane in an identical fashion as it will be in full operation as adisplay. There need be no connection to L1 since there is no OLED D1 yetintegrated with the back plane. This calibration process is describedrelative to the second embodiment of the pixel circuitry illustrated anddescribed relative to FIG. 8, where capacitor C2 is charged through thesensor line and Vcap.

First (Step 801), the active-matrix backplane (am backplane) is loadedinto the test fixture which is connected to the display control system,such as for example the control system illustrated in FIG. 4.

Second (Step 802), the am backplane is uniformly illuminated with acalibrated laboratory uniform light source at a luminance equal to grayscale luminance 1. (This step may be performed with the backplaneuniformly illuminated with a light source at a luminance equal to grayscale luminance of a different level, such as another low levelillumination so long as the level is known and the calibration proceduretakes this different level into account, but this approach is notpreferred.)

Third (Step 803), the display controller Z104 sends a select row 1 toRow Select 240 to turn on all the T3 transistors in row 1 of thedisplay.

Fourth (Step 804), since the third step (Step 803) turned on all thetransistor T3s in row 1, and charge flows from the sensor line L4 intocapacitor C2 charging it to a voltage, such as for example, charging itto 10 volts.

Fifth (Step 805), when capacitor C2 charges, the current is sensed byoperational amplifier (OP amp) to generate VC1 and the value is sampledand held by Z101 for each pixel in row 1.

Sixth (Step 806), the sampled and held voltages are digitized andmultiplexed (MUX) to a serial data stream by A/D Converter 207 and MUX,207 a. The sequence of the D/A and MUX may be interchanged with noaffect on performance.

Seventh (Step 807), the display controller Z104 directs the serial datastream to be stored as the zero line to Calibration Memory (Cal Mem)250. This is referred to as the zero line because this data is onsensors that have not had the full frame time to photon flux integratethe gray level.

Next (Step 808), Steps 803 through Steps 807 are repeated for all rowsin the display to be calibrated (usually every row) until all rows inthe frame have been sampled. At this point the first gray level ofemission for the first row has been integrated by S1 and C2 for the fullframe time.

After all rows have been calibrated for the gray level 1 value, the nextstep (Step 809) repeats Steps 803 though Step 807 for the next graylevel to be calibrated, usually gray level 2 in the preferredembodiment. The sample and held values determined from Step 806 are theproper values for the first gray level and are stored in Step 801 to thefirst row values for gray level 1.

In a final step (Step 810), each of the first nine steps (Step 801through Step 809) are repeated until all gray levels have been sampledand stored to Calibration Memory (Cal Mem) 205. Note that in oneembodiment, the last or highest gray level (e.g. gray level 256 for an8-bit system) may be or is run for two frames since the gray levelvalues recorded at the beginning of the 256^(th) frame are for the255^(th) gray level and this assures that the final value is stored inthe Calibration Memory 250.

The second embodiment for calibration (post-manufacture calibration),calibrates the completed manufactured display, such as for example whenthe display is first powered on, booted-up, or otherwise initialized orused for the first time. This calibration system assumes that themanufacturer adjusted the display in the usual manner prior to shipmentfor sale to the display user or OEM manufacturer of another device.Therefore the voltages used to operate the display have been put into agamma table or other look-up table as is the usual practice in theindustry. This means that the first sensor values measured areautomatically calibrated. This embodiment takes advantage of themanufacturer's calibration. Details of this post-manufacture calibrationare described with reference to the embodiment illustrated in FIG. 10.

First (Step 831), the analog image Data function logic block 208 sendsfirst gray level 1 image voltage for the first pixel (pixel 1) in thefirst row (row 1) to image A/D converter 209 where the analog voltage isdigitized to a gray level 1 digital value. (Where the gray level imagevalues are already in digital form this analog-to-digital conversion isnot necessary.)

Second (Step 832), this digitized gray level 1 voltage value is sent orotherwise communicated to gray level logic function block Z103.

Third (Step 833), gray level logic function block Z103 combinesinformation from (i) the manufacturer's (or an otherwise generated oravailable) gamma table Z103 b and from (ii) a pixel deviation memoryZ102 within a pixel deviation logic block, but since there are no valuesyet stored or only default values stored in the pixel deviation memorythere is no change to the manufacturer's value determined by the gammatable. (The pixel deviation logic block and the pixel deviation memoryand its stored values are described in greater detail herein below.)

Fourth (Step 834), the digital gray level 1 voltage is sent to DisplayController function logic block Z104.

Fifth (Step 835), Display Controller function logic block Z104 relaysthe digital gray level 1 voltage value to display first column driver(column driver 1) in function logic block 238.

Sixth (Step 836), Step 831 through Step 835 are repeated for all thepixels in the first row until all the pixels data in row 1 have beenloaded into a line buffer in column driver 238.

Seventh (Step 837), on command from Display Controller Z104, the row 1pixel data is downloaded to a series of digital-to-analog converters(DACs) at the head of each column, where each digital pixel voltage isconverted to an analog voltage and loaded onto the line L3s for eachcolumn of pixels.

Eighth (Step 838), display controller Z104, after waiting for the analogvoltages on the column lines L3 to stabilize, sends a select row 1signal to the Row Select function logic block 240.

Ninth (Step 839), the row select function logic block 240 puts a highvoltage on line L2 and turns on all the gates to all the transistor T1in row 1, causing the display voltage on line L3 to flow into capacitorC1 where it is held when the voltage on line L2 goes low; and at thesame time transistor T3 is turned on causing charge to flow intocapacitor C2 from sensor line L4.

Tenth (Step 840), the movement of charge into capacitor C2 causes avoltage to be sampled and held in function logic block Z101, and a valuefor each individual sensor S1 in row one is read.

Eleventh (Step 841), the sample and held voltages are digitized andmultiplexed (or multiplexed and then digitized) to a serial data streamby A/D Converter 207 and multiplexer (MUX) 207 a.

Twelfth (Step 842), Display Controller Z104 directs the serial sensordata stream to be stored in row 1 of Calibration Memory (Cal Mem) 250for gray level zero.

Thirteenth (Step 843), Step 836 through Step 843 are repeated until allrows in the frame have been sampled and stored for gray level 0.

Fourteenth (Step 844), Step 831 through Step 843 are repeated for graylevel 2. The sensor values read on this frame are for the previous graylevel 1 and are stored in calibration memory (Cal Mem 250) as the valuesfor the first gray level or gray level 1.

Fifteenth (Step 845), Steps 831 through Step 844 are repeated until allgray levels have been sampled and saved to the calibration memory CalMem 250. Note that as in the pre-manufacture calibration procedure, thelast gray level is run for two frames so that the final value is storedin calibration memory Cal Mem 250.

The Pixel Deviation memory has been referred to in the above calibrationprocedures. In one embodiment, the Pixel Deviation memory stores data orother information that indicates changes, differences, history, aging orother data or information relevant to display operation and calibration.There are many methods to use the data such as aging data stored inPixel Deviation Memory Z102.

In one embodiment, for example, the voltage can be raised for the agedpixels that have undergone a decrease in luminance to bring them back tothe correct ruminations. One possible drawback in some embodiments maybe that voltage head room has to be built into the column drivers inColumn Drivers 238 to fully utilize this type of correction orcompensation. In another embodiment, another way to use the data inPixel Deviation Memory is implemented to reduce the number of graylevels for the less aged (or less degraded) pixels. Yet another methodis to use a 9-bit gray scale in a nominally 8-bit system allowing thehighest gray level to increase beyond gray level 256 so that an agedpixel can be effectively be driven to level 257 (or other required graylevel value) so that it will emit a luminance at the luminance levelspecified for a gray level 256. Therefore, all the image gray levels forthat pixel would be bumped up by one (or an appropriate number) level ofgray. Another method uses spatial dithering, a well know gray scalemethod, to increase the effective number of gray levels withoutincreasing the number of bits in the logic. Alternately, temporaldithering which is known for conventional displays may be used, orcombinations of spatial and temporal dithering can be used. Thesedifferent methods or techniques and the structures associated with suchmethods may be used alone or in any combination with each other or withother techniques.

4. Embodiment of the Sensor Read Circuit and Method

FIG. 5 shows an exemplary embodiment of a voltage sensing amplifier readcircuit. When the row is selected by Row Select 240 the voltage goeshigh on line L2 turning on transistor T3 allowing voltage on capacitorC2 to transfer to the plus terminal on operational amplifier OA1. Thisvoltage is amplified according to the ratio of resistance R2 to theresistance (R_(L4))of line L4. Typically the resistance R_(L4) of L4 ison the order of several kilo-ohms (≈10³ ohms). Therefore, if resistanceR2 is several megohms (≈10⁶ ohms) the amplification factor is 30 dB or1000 to 1. Therefore, a one-millivolt reading on capacitor C2 would showup on pin or node P4 as one-volt and be sent to the sample and holdfunction block Z101. One possible drawback to this circuit is that anyparasitic capacitance on line L4 may reduce the voltage on capacitor C2during the read time. Therefore this circuit is best used for a displaywith a low number of rows and therefore a relatively low resolutiondisplay, but in any event even with this possible limitation,performance relative to conventional circuits and methods is improvedand this potential constraint is only pointed out so that the virtues ofa second embodiment may be appreciated to the fullest.

The second embodiment of the read circuit is shown in FIG. 6 and istermed a charge amp/transimpedance amplifier. It gets its name from thefact that the charge required to re-charge capacitor C2 to the fullvoltage is measured by this circuit and that the input of the circuit(the negative input on charge amplifier CA1) is in the Gig-ohms range orhigher and the output at pin or node P3 is almost zero ohms. In fact thenode at P3 may sometimes be viewed as a virtual ground.

Operation of this embodiment of the circuit is now described withreference to FIG. 6. A voltage is placed on the plus input pin of firstcharge amplifier CA1, for example, 10 volts (or other establishedvalue). Since initially there is no voltage on the negative input pin,10 volts instantly appears on pin P3 and is transferred to the negativeinput pin by C3. Subsequently, the now 10 volts appearing on thenegative pin is subtracted from the 10 volts on the positive input pinof the first charge amplifier CA1, causing the voltage on pin P3 tobecome zero (or substantially zero), but the 10 volts on the negativepin remains, because if the voltage on the negative input pin decays byan amount of voltage (such as by a volt, for example) then this voltagedifference (a volt) shows up on pin P3 thereby boosting the voltage onthe negative pin back up to 10 volts (or other established value). Thisis similar to how a charge pump works.

When the circuit settles, there is 10 volts (or other established value)on both the input pin to charge amplifier CA1 and zero volts on pin P3.Node or pin P3 may almost never be exactly 0 volts for a couple ofreasons. First of all, the family of operational amplifiers to whichcharge amplifier CA1 belongs may typically have an offset voltage,because the pair of internal transistors that make up the operationaltransistor may not usually be exactly alike in characteristics orperformance and the difference shows up a the offset voltage. Anotherreason that the voltage on P3 is not zero is that L4 is connected to allthe T3 transistors in the column. This may for example, be as high asone-thousand T3 transistors for a high resolution display having 1000rows, and an even greater number for larger and/or higher resolutiondisplays. Each of these T3s may typically have a current leakage on theorder of several pico-amps (10⁻¹² amps) that tends to lower the voltageon the negative pin of the charge amplifiers CA1 causing a voltage toappear at pin P3 on top of the afore described offset voltage. Inoperation, the voltage on pin P3 is sampled before the voltage on L2goes high, in order to determine the voltage caused by the offsetvoltage and the leakage current on line L4. Pin P3 is advantageouslyagain sampled after the voltages on line L2 goes high and the twovoltage subtracted (using logic functions common in the industry) togenerate a difference voltage. The difference between the two readingsis a measure of the charge moving into capacitor C2 to bring line L4 andcapacitor C2 up to the 10 volts (or other established value) as used inthe example.

One advantage of this embodiment is that the reading of charge by avoltage change on pin P3 is independent (or substantially independent)of the capacitance on line L4. The first charge amplifier CA1 keeps lineL4 charged to the voltage on its plus (+) input pin. If one electron isremoved from L4 then one electron moves out of capacitor C3 to replaceit, and any movement of electrons from capacitor C2 cause the voltage todecrease on the negative input pin of charge amplifier CA1 with acorresponding voltage change on pin P3. In one embodiment, the value ofthe C3 capacitance is selected to be on the same order as the C2capacitance; therefore, if capacitor C2 has a capacitance of about apicofarad then C3 should also be selected to have a capacitance of abouta picofarad, but they need not have identical values. The chargeamplifier may be a typical operational amplifier as used in theindustry. The size of the charge amplifier (its power rating) isdetermined by taking into account the leakage on line L4. If for examplethe leakage of one-thousand T3 transistors is a several nano-amps, thencharge amplifier CA1 is advantageously able to supply several nano-amps,and preferable this amount with some safety margin. Embodiments of theinvention provide safety margins of a factor of two or three times theleakage current, but lesser or greater safety margins may beimplemented.

The discussion has focused on the inventive sensor circuit and itsoperation. It will be appreciated that any photoconductive (orphotoresistive) material may be used for the sensor, including forexample any of amorphous silicon, poly-silicon, cadmium selenide, orother photoconductive or photoresistive materials that are know in theart or to be developed in the future. It will also be appreciated that apoly-silicon based sensor may provide for an inherently more stabileoperation than an amorphous silicon based sensor, the use ofpoly-silicon also has inherently greater production costs for a displaybecause the flat panel display manufacturing infrastructure is wellestablished for amorphous silicon, but would need to be rebuilt forpoly-silicon at costs measured in the billions of dollars. Therefore theinventive system, structure, and method that permit use of amorphoussilicon materials through its calibration and feedback stabilization andcontrol provide distinct advantages. Issues associated with thedifferences between crystalline silicon (x-Si), poly-silicon (p-Si) andamorphous silicon (a-Si) are described elsewhere in this specification.

5. Embodiment of Method of Operation the Display Device and System

Having described many features of the inventive system and device andcalibration methods and techniques related thereto, further attention isdirected to aspects of operation of the display. Attention is focused onembodiments that use the read circuit of the FIG. 6 embodiment, and thepixel circuit of the FIG. 8 embodiment with VC1 and T4 being replacedwith charge amplifier CA1 in FIG. 6. It will be apparent to thoseworkers having ordinary skill in the art in light of the descriptionprovided here that other combinations of the different embodimentsalready described may be utilized for the display device and system.

An embodiment of a system and method for operating a display and displaysystem is now described with reference to the flow-chart diagram of FIG.11. This sequence of steps is exemplary, including optional steps, andit will be apparent that some reordering of steps may be made, and thatother steps may be performed in parallel, without deviating from thespirit and scope of the invention.

First (Step 851), the analog image Data function logic block 208 sendsthe image voltage for the first pixel in the first row (pixel 1, row 1)to the image A/D Converter 209 where the image analog voltage isconverted to a digital number representing the image gray level, which,in an 8-bit gray level system, is a number between 0 and 255. For a graylevel system supporting a different number of bits of pixel gray leveldata, the digital number will correspond to that range or to a lesserrange if fewer than all possible levels are actually utilized. An 8-bitgray level system with 256 levels for each color channel will be assumedfor purposes of this description, but this in no way limits theinvention. (Note that performing this procedure or any of the otherprocedures beginning with the first pixel of the first row and thensubsequent pixels of the first row and then all the other rows makeslogical sense, but neither this procedure requires this starting pointor sequence, and in reality so long as the logic is designed tocalibrate and/or operate each pixel in the described manner, anyordering may be used.)

Second (Step 852), this image gray level value between 0 and 255 is sentto the gray level logic function block Z103.

Third (Step 853), gray level logic function block Z103 converts the graylevel number for the first pixel in the first row (pixel 1, row 1) intoa digital voltage to be applied to the pixel to cause the OLED D1 toemit a photon flux at a level of luminance to equal the image gray levelinput to the display system at the first step. This voltage isdetermined using information in the manufacturer's gamma table and theinformation from the Pixel Deviation Memory Z102. Initially when thedisplay is new there is no deviation data in Pixel Deviation Memory orthe values stored there are default values so that these values will notreally change the manufacturer's gamma table values, but as the displayages pixel deviation values are built up in the Pixel Deviation MemoryZ102.

Fourth (Step 854), the digital voltage for the first pixel of the firstrow (pixel 1, row 1) is sent to Display Controller Z104.

Fifth (Step 855), Display Controller Z104 relays or otherwisecommunicates a digital voltage for the first pixel in the first row(pixel 1, row 1) to a line buffer in Column Driver 238. Line buffers fordisplays are known in the art and not described here in greater detail.The pixel voltage for the first pixel in the first row (pixel 1, row 1)is loaded into the line buffer at the first column position (columnposition 1).

Sixth (Step 856), Step 851 through Step 855 are repeated for all thepixels in the row until all the pixels in the row have voltages loadedinto the line buffer of Column Driver 238.

Seventh (Step 857), on command from the Display Controller Z104, thefirst row (row 1) pixel data is downloaded to a series of parallel DACs(one for each column in the display) which convert the digital pixelvoltages to analog voltage applied to all the L3 (one for each column).

Eighth (Step 858), Display controller Z104, after waiting for thevoltage placed on L3 to settle sufficiently, sends a select row 1 signalto Row Select logic block 240.

Ninth (Step 859), Row Select logic block 240 places a high voltage online L2 for row 1, therefore turning on all T1 transistors in the firstrow and causing the voltages applied to the line L3s to be transferredto the C1 capacitors in all the pixels in the first row. This in turn,causes the power TFT transistor T2 to supply current to the D1 OLEDdiodes in the first row. At the same or substantially the same time, allthe sensor TFTs T3 are turned on causing charge to flow into capacitorC2 until capacitor C2 is at the re-charge voltage, for example, the 10Vexemplary value described in the earlier example.

Tenth (Step 860), the movement of charge into capacitor C2 causes avoltage to be sampled and held in function logic block Z101 for eachpixel in the row. Eleventh (Step 861), the sampled and held voltages aredigitized and multiplexed to a serial data stream (the order ofdigitizing and multiplexing can be reversed without loss of performance)by A/D Converter 207 and multiplexer MUX 207 a.

Twelfth (Step 862), Display Controller Z104 directs the serial sensordata and a stream of calibration data from Calibration Memory (Cal Mem)250 to meet at Comparator 260 so that a comparison of the serial sensordata and the calibration data for the pixels can be generated.

Thirteenth (Step 863), comparator 250 subtracts (or generates adifference between) the sensor data from the calibration data and sendsthe result to Pixel Deviation Memory Z102 for the first row (row 1)where the data is stored according to pixel number and row (or any otherscheme) and by gray level established for the pixel in the first step(Step 851) that is a digital number representing the image gray level.

Fourteenth (Step 864), Step 856 through Step 863 are repeated for allrows until all rows in the frame have been down loaded and deviations(if any) have been determined and stored in Pixel Deviation Memory.

Fifteenth (Step 865), Step 851 through Step 864 are repeated for eachframe (or for any designated frame according to an established plan ofoperation). While one embodiment performs the procedure for each frame,this is not necessary as pixels do not normally age or otherwise changeat this rate. Alternatives may include repeating the procedure at anypredetermined number of frames, at device power-on, after a clockdetermined period of time of operation, in response to an automaticallyor manually generated signal, or other event. In one embodiment, theprocedure is repeated for every frame as once the circuits and methodshave been established, there is no cost in performing the procedure forevery frame.

6. Embodiment of a Display System

FIG. 12 is an illustration showing an embodiment of a display systemaccording to aspects of the present invention. A display screen 602having a plurality of emissive pixels 603 of the type already describedarranged in an array is held or mounted within a housing 604 such as amonitor frame, cabinet, or other device, and displays an image 605 orother two dimensional graphic. (Note that one-dimensional displays mayalso be fabricated using the features of the invention but althoughpossibly useful are less interesting.)

Circuits and devices that are formed on the display substrate (oftenglass or polymeric material) are referred to as on-glass circuits anddevices while those that are not formed on the display substrate arereferred to as off-glass circuits and devices. The pixels including thepixel emitters D1, sensors S1, sensor capacitors C2, and other elementsformed within each of the display pixels are formed on-glass. Otherelements may be formed off glass according to conventional displaydesign principles. The on-glass circuits and devices connect to theoff-glass circuits and elements such as display drive and controlelectronics 606 over an interface 608. These display drive and controlelectronics 606 may be mounted within or without the monitor housing 604but may usually be housed within so that a user may simply plug in oneor more (analog or digital) video or image sources (such as for example,a DVD player 610, a computer 612, a video or digital camera 614, ormemory card 616) and have the image or video displayed. Alternatively orin addition, the display system 600 may include image generators withinthe system, such as a TV tuner or receiver 618 or other internalgenerator. Of course there may be various other wired or wirelessinterfaces for sending data to the system 600 for display. A switchingdevice SW 620 may be provided to manually or automatically select whichof the sources are to be displayed, and multiple sources may besimultaneously displayed such as by using picture-in-picture technology.The system may also support various forms of image processing andenhancement.

This is only one example of the application of the display technology toimaging applications and it will be appreciated that although a primaryapplication of the technology is to flat-panel displays, the inventivetechnology may be applied to displays having curved surfaces as well.There are an endless variety of display applications for which theinventive technology may be applied. We list several by way of examplebut not limitation; they include: any information appliance, atelevision monitor, a CD player, a DVD player, a computer monitor, acomputer system, an automobile instrument panel, an aircraft instrumentdisplay panel, a video game, a cellular telephone, a personal dataassistant (PDA), a telephone, a graphics system, a printing system, ascoreboard system, document and image scanners, an entertainment system,a domestic or home appliance, a copy machine, a global positioningsystem navigation display, a dynamic art display device, a digital orvideo camera, and any combinations of these.

7. Exemplary Embodiments Having Particular Combinations of Features

Various structures, devices, systems, architectures, methods,procedures, and computer programs have been described in thisspecification and illustrated in the figures. It will be appreciated inlight of the description that the invention provides many differentfeatures and elements that can be utilized separately or in variouscombinations. This section of the description sets forth some particularembodiments that have or require particular combinations of features andelements of the invention. The combinations set forth are merelyexemplary, and any of the features and elements described in thissection or in the specification as a whole may be used separately or incombination. It will also be appreciated that the section headers andsub-headers set forth in the detailed description are merely intended toserve as a guide to the reader and that different aspects, features, andelements of the invention are set forth throughout the specification.

In one aspect the invention provides a system and method for a long-lifeluminance feedback stabilized display panel. In a first embodiment, theinvention provides a stabilized feedback display system comprising: adisplay device having a plurality of emissive picture elements (pixels)each formed from at least one electronic circuit device; a displaydriver circuit receiving a raw input image signal from an external imagesource and applying a corrected image signal the display; a displayluminance detector generating at least one display device luminancevalue; and a processing logic unit receiving the at least one displaydevice luminance value and communicating information to the displaydriver circuit, the display driver circuit using this communicatedinformation to generate a transformation for generating the correctedimage signal from the raw input image signal.

In second particular embodiment of this system, each of the pictureelements comprises: a sample and hold circuit; a current sourcecontrolled by the sample and hold circuit; a photon emission devicesupplied by the current source; and a luminance detection devicedisposed within a separation distance from the photon emission devicefor detecting photons emitted by the photon emission device.

In a third embodiment, each of the picture elements comprises: a photonemitter; and a photon flux integrator disposed within the pixel tointercept a flux of photons from the photon emitter during a specifiedtime, to undergo an electrical property change in response to thephotons intercepted, to integrate or count the number of photonsintercepted during the time, and to generate a signal indicative of athe total integrated photon flux during the specified time. In a fourthembodiment, the photon flux integrator comprises: a sensor formed of aphoto device that exhibits changing or variable properties in responseto a changing or variable photon flux; a charge storage device adaptedto store or release charges; and a control circuit that directs chargesto or removes charges from the charge storage device in response to thechange in resistance or conductance of the sensor. In a fifthembodiment, the charge storage device comprises a capacitor. In a sixthembodiment, the control circuit includes a transistor. In a seventhembodiment, the photo device comprises a photo sensitive resistor thatchanges its resistance or conductance with changes of photon fluximpinging on its surface. In an eighth embodiment, photo devicecomprises a photo diode the leakage of which increases or decreases withvariations of photon flux impingent on its surface. In a ninthembodiment, the photo diode leakage comprises one or more of voltageleakage, current leakage, or charge leakage. In a tenth embodiment, thephoto device comprises a phototransistor the current of which increasesor decreases with variations of photon flux impingent on thephototransistor surface.

In another embodiment of the system, the luminance detector comprises aphoton flux integrator. In another embodiment of the system, the pictureelement (pixel) comprises a particular photon flux integrator thatintegrates a photon flux emitted by the photon emission device withinthe same pixel as the photon flux integrator. In another embodiment ofthe system, each photon flux integrator comprises: an isolationswitching device for isolating a first circuit node from a secondcircuit node and having an output port (node); a photosensitive unithaving an input coupled to the isolation switching device output port(node) and an output connected with a voltage reference node; and acharge storage device having a first electrode coupled with a first portof the isolation switch and a second electrode coupled with the voltagereference node. In another embodiment of the system, the charge storagedevice comprises a capacitor. In another embodiment of the system, theisolation switch comprises a transistor. In another embodiment of thesystem, the isolation switch is formed on a substrate as a thin filmtransistor (TFT). In another embodiment of the system, the thin filmtransistor is constructed from amorphous silicon. In another embodimentof the system, the thin film transistor is constructed from polysilicon.In another embodiment of the system, the thin film transistor isconstructed from cadmium selenide. In another embodiment of the system,the thin film transistor is constructed from any semiconductor material.

In another embodiment of the system, the thin film transistor comprisesa channel defined in a material, and the material is selected from theset of materials consisting of: an amorphous silicon channel, apoly-silicon channel, a cadmium selenide channel, a gallium arsenidechannel, and a channel formed or defined in any other semiconductingmaterial.

In another embodiment of the system, the display device comprisesmultiple picture elements arranged in a planar array. In anotherembodiment of the system, the multiple individual picture elements areaddressed by column and row. In another embodiment of the system, thespecified time is equal to or less than the row address time. In anotherembodiment of the system, the specified time is between 0.01 (1 percent)of the row address time and the row address time. In another embodimentof the system, the specified time is between 0.1 (10 percent) of the rowaddress time and the row address time. In another embodiment of thesystem, the specified time is equal to or less than the frame time. Inanother embodiment of the system, the specified time is greater than0.01 of the row address time and less than or equal to the frame time.In another embodiment of the system, the specified time is equal tomultiple frame times.

In another embodiment of the system, the display emissive device is anorganic light emitting diode (OLED). In another embodiment of thesystem, the organic light emitting diode (OLED) is a small moleculeOLED. In another embodiment of the system, the organic light emittingdiode (OLED) is a polymer OLED (PLED). In another embodiment of thesystem, the organic light emitting diode (OLED) is a phosphorescent OLED(PHOLED). In another embodiment of the system, the organic lightemitting diode (OLED) is constructed from any organic material in anycombination of single or multiple layers of organic materials andelectrodes. In another embodiment of the system, the organic lightemitting diode (OLED) is a active matrix OLED. In another embodiment ofthe system, the display emissive device is an electroluminescent device.In another embodiment of the system, the display emissive device is aplasma emission device. In another embodiment of the system, the displayemissive device is any controllable photon emissive device. In anotherembodiment of the system, the active matrix is constructed fromamorphous silicon. In another embodiment of the system, the activematrix is constructed from poly silicon. In another embodiment of thesystem, the active matrix is constructed from cadmium selenide. Inanother embodiment of the system, the active matrix is constructed fromany type of semiconductor material.

In another aspect, the invention provides a method for stabilizing adisplay system comprising: providing a display device having a pluralityof emissive picture elements (pixels) each formed from at least oneelectronic circuit device; receiving a raw input image signal by adisplay driver circuit from an external image source and applying acorrected image signal to the display; detecting a display luminance andgenerating at least one display device luminance value; and receivingthe at least one display device luminance value by a processing logicunit and communicating information to the display driver circuit, andusing this communicated information to generate a transformation forgenerating the corrected image signal from the raw input image signal.

In another aspect the invention provides a method for operating andindividually controlling the luminance of each pixel in an emissiveactive-matrix display device. In one embodiment of this method, theinvention provides a method for controlling the luminance of a pictureelement (pixel) in a display device, the method comprising: storing atransformation between a digital image gray level value and a displaydrive signal that generates a luminance from a pixel corresponding tothe digital gray level value; identifying a target gray level value fora particular pixel; generating a display drive signal corresponding tothe identified target gray level based on the stored transformation anddriving the particular pixel with the drive signal during a firstdisplay frame; measuring a parameter representative of an actualmeasured luminance of the particular pixel at the end of the firstdisplay time; determining a difference between the identified targetluminance and the actual measured luminance for the particular pixel;modifying the stored transformation for the particular pixel based onthe determined difference; and storing and using the modifiedtransformation for generating the display drive signal for theparticular pixel during a frame time following the first frame time.

In another embodiment of this method, the first display frame is anydisplay frame designated by software programming or by the display useror by a combination of the programming and the user. In anotherembodiment of this method, the frame time following the first frame isany subsequent frame time. In another embodiment of this method, thefirst display frame is any display frame designated by softwareprogramming or by the display user or by a combination of theprogramming and the user. In another embodiment of this method, thefirst display time may be either a single continuous period of time orcomprised of a plurality of discontinuous periods of time, and whereineither of the continuous period of time and the discontinuous periods oftime may occur during a single frame time or over multiple frame times.In another embodiment of this method, storing and/or the using of themodified transformation for generating the display drive signal for theparticular pixel is applied at any subsequent portion of a single frameor at different frames. In another embodiment of this method, thestoring and/or the using of the modified transformation for generatingthe display drive signal for the particular pixel may be either atsingle continuous period of time or comprised of a plurality ofdiscontinuous periods of time, and wherein either of the continuousperiod of time and the discontinuous periods of time may occur during asingle frame time or over multiple frame times. In another embodiment ofthis method, the storing and/or the using of the modified transformationfor generating the display drive signal for the particular pixel may beeither at single continuous period of time or comprised of a pluralityof discontinuous periods of time, and wherein either of the continuousperiod of time and the discontinuous periods of time may occur during asingle frame time or over multiple frame times.

In another embodiment of this method, the stored transformationcomprises a transformation stored in a gray level logic functional blockof a display system. In another embodiment of this method, the storedtransformation comprises a transformation stored in a gamma table for adisplay device. In another embodiment of this method, the measuredparameter representative of an actual measured luminance of theparticular pixel at the end of the first display time comprises avoltage measurement corresponding to a number of electrons accumulatedor released from a charge storage device. In another embodiment of thismethod, the measured parameter representative of an actual measuredluminance of the particular pixel at the end of the first display timecomprises a current measurement corresponding to a number of electronsaccumulated or released from a charge storage device. In anotherembodiment of this method, the measured parameter representative of anactual measured luminance of the particular pixel at the end of thefirst display time comprises a charge measurement corresponding to anumber of electrons accumulated or released from a charge storagedevice. In another embodiment of this method, the charge storage devicecomprises a capacitor. In another embodiment of this method, theelectrons are accumulated or released in proportion to a resistivity orconductivity of a sensor element having a resistivity or conductivitythat changes in response to a flux of photons incident on the sensor. Inanother embodiment of this method, the proportion is a directproportion.

In another embodiment of this method, the frame time following the firstframe time is the next subsequent frame time. In another embodiment ofthis method, the frame time following the first frame time is anysubsequent frame time. In another embodiment of this method, the frametime following the first frame time is a next display device power ontime. In another embodiment of this method, the frame time following thefirst frame time is a frame time at a predetermined or dynamicallydetermined time interval. In another embodiment of this method, adifferent transformation is stored for each pixel in the display device.In another embodiment of this method, a different transformation isstored for each different gray level that may be displayed for eachseparately addressable pixel in the display device. In anotherembodiment of this method, the first display time is the duration oftime a pixel is turned on in the display. In another embodiment of thismethod, the display time is substantially any time between 8milliseconds and 36 milliseconds. In another embodiment of this method,the display time is substantially any time between 10 milliseconds and20 milliseconds. In another embodiment of this method, the portion ofthe frame time comprises substantially the row address time. In anotherembodiment of the method, the specified time is equal to or less thanthe row address time. In another embodiment of the method, the portionof the frame time is between 0.01 (1 percent) of the row address timeand the row address time. In another embodiment of the method, theportion of the frame time is between 0.1 (10 percent) of the row addresstime and the row address time. In another embodiment of the method, theportion of the frame time is equal to or less than the frame time. Inanother embodiment of the method, the portion of the frame time isgreater than 0.01 of the row address time and less than or equal to theframe time. In another embodiment of the method, the portion of theframe time is equal to multiple frame times. In another embodiment ofthe method, the portion of the frame time comprises a time between therow address time and the frame time.

In another embodiment of the method, the measuring of a parameterrepresentative of an actual measured luminance of the particular pixelat the end of the first display time comprises measuring a voltagestored on a capacitor that has either been charged toward or dischargedfrom a known voltage and the amount of charging or discharging isproportional to a photon flux emitted from the emitter within theparticular pixel onto a sensor within the same particular pixel.

In another embodiment of the method, the steps of identifying,generating, measuring, determining, modifying, and using are repeatedfor every pixel in the display. In another embodiment of the method, thedetermining of a difference between the identified target luminance andthe actual measured luminance for the particular pixel is based on areference integrated photon flux on the particular pixel sensordetermined during a display calibration procedure performed duringmanufacture or when initially used. In another embodiment of the method,the method further comprising a display calibration procedure thatdetermines and stores an initial transformation for every pixel andevery gray level the display may be commanded to display.

In another aspect the invention provides a control system forcontrolling the luminance of a picture element (pixel) in a displaydevice, the control system comprising: a stored pixel gray level todisplay pixel drive signal transformation for each pixel and each graylevel the pixel may be commanded to display, the stored transformationbased on performance characteristics of the display pixels during aprior display frame time period; a display drive signal generatorresponsive to a control that receives a command to display a particulargray level for a particular pixel location and generates a drive signalto the particular pixel using the stored transformation during a firstframe time; a luminance measurement circuit for each separate pixel inthe display for measuring parameters representative of an actualmeasured luminances of each of the plurality of particular pixels at theend of the first display time; a comparator circuit for determining adifference between the identified target luminance and the actualmeasured luminance for the particular pixel; transformation update logicfor modifying the stored transformation for each particular pixel basedon the determined difference during a portion of a first frame time; andusing the modified transformation for generating the display drivesignal for the particular pixel during a portion of a second frame timefollowing the first frame time.

In another embodiment of the control system, the stored transformationcomprises a transformation stored in a gray level logic functional blockof a display system. In another embodiment of the control system, thestored transformation comprises a transformation stored in a gamma tablefor a display device. In another embodiment of the control, system, theluminance measurement circuit measures a parameter representative of anactual measured luminance of the particular pixel at the end of thefirst display time and comprises a voltage measurement corresponding toa number of electrons accumulated or released from a charge storagedevice separately for each pixel of the display. In another embodimentof the control system, the charge storage device comprises a capacitor.In another embodiment of the control system, the electrons areaccumulated or released in proportion to a resistivity or conductivityof a sensor element having a resistivity or conductivity that changes inresponse to a flux of photons incident on the sensor. In anotherembodiment of the control system, the proportion is a direct proportion.In another embodiment of the control system, the second frame timefollowing the first frame time is a portion of the next subsequent frametime. In another embodiment of the control system, the portion of asecond frame time following the portion of the first frame time is aportion of time in any one or plurality of subsequent frame times. Inanother embodiment of the control system, the frame time following thefirst frame time is a next display device power on time. In anotherembodiment of the control system, the frame time following the firstframe time is a frame time at a predetermined or dynamically determinedtime interval. In another embodiment of the control system, a differenttransformation is stored for each pixel in the display device. Inanother embodiment of the control system, a different transformation isstored for each different gray level that may be displayed for eachseparately addressable pixel in the display device. In anotherembodiment of the control system, the first display time is the durationof time a pixel is turned on in the display.

In another embodiment of the control system, the display time issubstantially any time between 8 milliseconds and 36 milliseconds. Inanother embodiment of the control system, the display time issubstantially any time between 10 milliseconds and 20 milliseconds. Inanother embodiment of the control system, the portion of the frame timecomprises substantially the row address time. In another embodiment ofthe control system, the portion of the frame time comprises a timebetween the row address time and the frame time. In another embodimentof this control system, the portion of the frame time comprisessubstantially the row address time. In another embodiment of the controlsystem, the portion of the frame time is equal to or less than the rowaddress time. In another embodiment of the control system, the portionof the frame time is between 0.01 (1 percent) of the row address timeand the row address time. In another embodiment of the control system,the portion of the frame time is between 0.1 (10 percent) of the rowaddress time and the row address time. In another embodiment of thecontrol system, the portion of the frame time is equal to or less thanthe frame time. In another embodiment of the control system, the portionof the frame time is greater than 0.01 of the row address time and lessthan or equal to the frame time. In another embodiment of the controlsystem, the portion of the frame time is equal to multiple frame times.In another embodiment of the control system and method, the portion ofthe frame time comprises a time between 0.01 of the row address time andthe frame time.

In another embodiment of the control system, the measuring of aparameter representative of an actual measured luminance of theparticular pixel at the end of the first display time comprisesmeasuring a voltage stored on a capacitor that has either been chargedtoward or discharged from a known voltage and the amount of charging ordischarging is proportional to a photon flux emitted from the emitterwithin the particular pixel onto a sensor within the same particularpixel.

In another embodiment of the control system, the steps of identifying,generating, measuring, determining, modifying, and using are repeatedfor every pixel in the display. In another embodiment of the controlsystem, the determining of a difference between the identified targetluminance and the actual measured luminance for the particular pixel isbased on a reference integrated photon flux on the particular pixelsensor determined during a display calibration procedure performedduring manufacture or when initially used. In another embodiment of thecontrol system, the control system further comprises a displaycalibration procedure that determines and stores an initialtransformation for every pixel and every gray level the display may becommanded to display. In another embodiment of the control system, themeasured parameter representative of an actual measured luminance of theparticular pixel at the end of the first display time comprises acurrent measurement corresponding to a number of electrons accumulatedor released from a charge storage device. In another embodiment of thecontrol system, the measured parameter representative of an actualmeasured luminance of the particular pixel at the end of the firstdisplay time comprises a charge measurement corresponding to a number ofelectrons accumulated or released from a charge storage device. Inanother embodiment of the control system, the first display frame is anydisplay frame designated by software programming or by the display useror by a combination of the programming and the user. In anotherembodiment of the control system, the frame time following the firstframe is any subsequent frame time. In another embodiment of the controlsystem, the first display frame is any display frame designated bysoftware programming or by the display user or by a combination of theprogramming and the user. In another embodiment of the control system,the first display time may be either a single continuous period of timeor comprised of a plurality of discontinuous periods of time, andwherein either of the continuous period of time and the discontinuousperiods of time may occur during a single frame time or over multipleframe times.

In another embodiment of the control system, the storing and/or theusing of the modified transformation for generating the display drivesignal for the particular pixel is applied at any subsequent portion ofa single frame or at different frames. In another embodiment of thecontrol system, the storing and/or the using of the modifiedtransformation for generating the display drive signal for theparticular pixel may be either at single continuous period of time orcomprised of a plurality of discontinuous periods of time, and whereineither of the continuous period of time and the discontinuous periods oftime may occur during a single frame time or over multiple frame times.In another embodiment of the control system, the storing and/or theusing of the modified transformation for generating the display drivesignal for the particular pixel may be either at single continuousperiod of time or comprised of a plurality of discontinuous periods oftime, and wherein either of the continuous period of time and thediscontinuous periods of time may occur during a single frame time orover multiple frame times.

In another aspect the invention provides a feedback control system andmethod for operating a high performance stabilized active matrixemissive display. In one embodiment of this method, the inventionprovides a system for operating an active-matrix OLED display device orother emissive display device having a plurality of pixels, the systemcomprising: a gray level logic coupled to an external source of digitalimage data, the gray level logic including a transformation fortransforming a first representation of an image pixel gray level valueto a second representation of the same image gray level pixel value; adisplay controller operable to receive inputs from the gray level logicand to communicate image and control signals to display matrix rowselect and column drive circuits, the row select and column driversoperable to cause an image to be displayed during a frame time for aplurality of pixels; each of the plurality of pixels including a pixelphoton flux emitter and a pixel photon flux receptor that integrates atleast a portion of the emitted photon flux from the emitter during aportion of the pixel display frame time and generates an output signalindicative of the integrated photon flux; a calibration memory storing acalibration value for each pixel and each pixel value that may bedisplayed by the pixel; a comparator receiving the output signals fromeach of the plurality of pixels and the calibration memory and comparingthe received output signals with a like plurality of correspondingsignals from the calibration memory to compute a difference signal foreach pixel; and a pixel deviation logic receiving difference signalsfrom the comparator and directing a change in the gray level logictransformation for at least pixel locations and pixel gray level valuesthat have a difference between the calibration and the measured values.

In another embodiment of this system, the pixel deviation logic includesa pixel deviation memory for storing a deviations between a calibratedpixel luminance value and a measured pixel luminance value. In anotherembodiment of this system, the calibration values are voltage values andthe output signals indicative of the integrated photon flux arevoltages, and the comparator is a voltage comparison circuit. In anotherembodiment of this system, the calibration values are current values andthe output signals indicative of the integrated photon flux arecurrents, and the comparator is a current based charge amp/impedancetransformation circuit. In another embodiment of this system, thecalibration values are charge values and the output signals indicativeof the integrated photon flux are charges, and the comparator is acharge based comparison circuit. In another embodiment of this system,the calibration values are voltage values and the output signalsindicative of the integrated photon flux are charges, and the comparatoris a voltage comparison circuit.

In another embodiment of this system, the output signal indicative ofthe integrated photon flux are analog signals, and the system furthercomprising: a sample and hold circuit for sampling an analog signal as avoltage representing a per pixel integrated photon flux during theportion of the pixel display frame time and holding that sampled signalfor conversion to a digital value; an analog to digital converterconverting the sampled and held analog signals to digital values; and amultiplexer coupled to the analog-to-digital converter and receivingdigital values and communicating them to the comparator according to apredetermined format and order.

In another embodiment of this system, the output signal indicative ofthe integrated photon flux are analog signals, and the system furthercomprising: a sample and hold circuit for sampling an analog signal as avoltage representing a per pixel integrated photon flux during theportion of the pixel display frame time and holding that sampled signal;a multiplexer coupled to the sample and hold circuit and receiving thesampled and held analog values; and an analog to digital converterconverting the sampled and held analog signals received from themultiplexer and converting the analog values to digital values andcommunicating them to the comparator according to a predetermined formatand order.

In another embodiment of this system, the system further comprising theexternal source of digital image data. In another embodiment of thissystem, the external source of digital image data comprises either asource of digital image data, or the combination of an analog image dataand a image analog-to-digital converter.

In another embodiment of this system, the portion of the frame timecomprises the row address time or a shorter period of time. In anotherembodiment of this system, the portion of the frame time comprisessubstantially the entire frame time. In another embodiment of thissystem, the portion of the frame time comprises at least 50 percent ofthe entire frame time. In another embodiment of this system, the portionof the frame time comprises at least between 90 percent and 100 percentof the entire frame time. In another embodiment of this system, theportion of the frame time comprises at least 1 millisecond. In anotherembodiment of the control system, the portion of the frame timecomprises substantially the row address time. In another embodiment ofthe control system, the portion of the frame time comprises a timebetween the row address time and the frame time. In another embodimentof this control system, the portion of the frame time comprisessubstantially the row address time. In another embodiment of the controlsystem, the portion of the frame time is equal to or less than the rowaddress time. In another embodiment of the control system, the portionof the frame time is between 0.01 (1 percent) of the row address timeand the row address time. In another embodiment of the control system,the portion of the frame time is between 0.1 (10 percent) of the rowaddress time and the row address time. In another embodiment of thecontrol system, the portion of the frame time is equal to or less thanthe frame time. In another embodiment of the control system, the portionof the frame time is greater than 0.01 of the row address time and lessthan or equal to the frame time. In another embodiment of the controlsystem, the portion of the frame time is equal to multiple frame times.In another embodiment of the control system, the portion of the frametime comprises a time between 0.01 of the row address time and the frametime.

In another embodiment, the invention provides a method for operating anactive-matrix display device having a plurality of pixels, the methodcomprising: storing a calibration value for each pixel and each graylevel value that may be displayed by each of the pixels in a calibrationmemory; storing a transformation in a transformation memory fortransforming first representations of an image pixel gray level valuesto second representations of the same image gray level pixel values foreach pixel and each gray level that may be displayed by each of thepixels in the display; receiving first gray level representations ofimage pixel gray level values for a plurality of pixels from an externalsource; transforming the first gray level representations to anequivalent number of second gray level representations for each pixel inaccordance with the stored transformation; generating image data andcontrol signals for driving pixel elements in a matrix display deviceduring a present display frame time in accordance with the secondrepresentation of the image gray level pixel value; generating anintegrated photon flux signal for each of the plurality of pixels in thedisplay indicative of the integrated photon flux on each of theplurality of pixels in the display during a portion of the presentdisplay frame time; comparing the plurality of integrated photon fluxsignals for a commanded gray level and with the calibration values forthe same gray level for each pixel on a pixel-by-pixel basis andgenerating a plurality of comparison results indicating a differencebetween the commanded gray level and the measured gray level; andidentifying any deviation for each pixel based on the comparison resultsand directing a change in the stored transformation to be applied duringa subsequent display frame time for at least pixel locations and pixelgray level values that have a difference between the calibration and themeasured values.

In one embodiment of this method, the step of identifying any deviationincludes storing pixel deviations between a calibrated pixel luminancevalue and a measured pixel luminance value in a pixel deviation memory.

In one embodiment of this method, the calibration values are voltagevalues and the integrated photon flux values are voltages, and thecomparison includes a comparison of voltages. In one embodiment of thismethod, the calibration values are current values and the integratedphoton flux values are currents, and the comparison includes acomparison of currents. In one embodiment of this method, thecalibration values are charge values and the integrated photon fluxvalues are charges, and the comparison includes a comparison of charges.

In one embodiment of this method, the integrated photon flux values areanalog signals, and the method further comprising: sampling an analogsignal as a voltage representing a per pixel integrated photon fluxduring the portion of the pixel display frame time and holding thatsampled signal for conversion to a digital value; and converting theanalog sampled signal to a digital signal.

In one embodiment of this method, the integrated photon flux values areanalog signals, and the method further comprising: sampling an analogsignal as a charge representing a per pixel integrated photon fluxduring the portion of the pixel display frame time and holding thatsampled signal for conversion to a digital value; and converting theanalog sampled signal to a digital signal.

In one embodiment of this method, the integrated photon flux values areanalog signals, and the method further comprising: sampling an analogsignal as a current representing a per pixel integrated photon fluxduring the portion of the pixel display frame time and holding thatsampled signal for conversion to a digital value; and converting theanalog sampled signal to a digital signal.

In one embodiment of this method, the method further comprisinggenerating the first gray level representations of image pixel graylevel values for a plurality of pixels. In one embodiment of thismethod, the digital image data comprises either a digital image data, oran analog image data that is converted to a digital data by an imageanalog-to-digital converter. In one embodiment of this method, theportion of the frame time comprises a time less than or equal to the rowaddress time.

In one embodiment of this method, the portion of the frame timecomprises substantially the entire frame time. In one embodiment of thismethod, the portion of the frame time comprises at least 50 percent ofthe entire frame time. In one embodiment of this method, the portion ofthe frame time comprises at least between 90 percent and 100 percent ofthe entire frame time. In one embodiment of this method, the portion ofthe frame time comprises at least 1 millisecond. In another embodimentof the method, the portion of the frame time comprises a time betweenthe row address time and the frame time. In another embodiment of thismethod, the portion of the frame time comprises substantially the rowaddress time. In another embodiment of the method, the portion of theframe time is equal to or less than the row address time. In anotherembodiment of the method, the portion of the frame time is between 0.01(1 percent) of the row address time and the row address time. In anotherembodiment of the method, the portion of the frame time is between 0.1(10 percent) of the row address time and the row address time. Inanother embodiment of the method, the portion of the frame time is equalto or less than the frame time. In another embodiment of the method, theportion of the frame time is greater than 0.01 of the row address timeand less than or equal to the frame time. In another embodiment of themethod, the portion of the frame time is equal to multiple frame times.In another embodiment of the method, the portion of the frame timecomprises a time between 0.01 of the row address time and the frametime.

In another embodiment of the method, the subsequent display frame timeis the next display time following the present display frame time. Inanother embodiment of the method, the subsequent display frame time isany display frame time following the present display frame time. Inanother embodiment of the method, the subsequent display frame time is aframe time at display initialization or power-on. In another embodimentof the method, the image data and control signals include display matrixrow and column control and drive signals operable to cause an image tobe displayed during a frame time for a plurality of pixels.

In another embodiment of the method, the pixels include at least onethin film transistor constructed from amorphous silicon. In anotherembodiment of the method, the pixels include at least one thin filmtransistor constructed from polysilicon. In another embodiment of themethod, the pixels include at least one thin film transistor constructedfrom cadmium selenide. In another embodiment of the method, the pixelsinclude at least one thin film transistor constructed from semiconductormaterial.

In another embodiment of the method, the portion of the present displayframe time is equal to or less than the row address time. In anotherembodiment of the method, the portion of the present display frame timeis equal to or less than the frame time. In another embodiment of themethod, the portion of the present display frame time is equal tomultiple frame times. In one embodiment of this method, the portion ofthe frame time comprises substantially the entire frame time. In oneembodiment of this method, the portion of the frame time comprises atleast 50 percent of the entire frame time. In one embodiment of thismethod, the portion of the frame time comprises at least between 90percent and 100 percent of the entire frame time. In one embodiment ofthis method, the portion of the frame time comprises at least 1millisecond. In another embodiment of the method, the portion of theframe time comprises a time between the row address time and the frametime. In another embodiment of this method, the portion of the frametime comprises substantially the row address time. In another embodimentof the method, the portion of the frame time is equal to or less thanthe row address time. In another embodiment of the method, the portionof the frame time is between 0.01 (1 percent) of the row address timeand the row address time. In another embodiment of the method, theportion of the frame time is between 0.1 (10 percent) of the row addresstime and the row address time. In another embodiment of the method, theportion of the frame time is equal to or less than the frame time. Inanother embodiment of the method, the portion of the frame time isgreater than 0.01 of the row address time and less than or equal to theframe time. In another embodiment of the method, the portion of theframe time is equal to multiple frame times. In another embodiment ofthe method, the portion of the frame time comprises a time between 0.01of the row address time and the frame time.

In another embodiment of the method, the display device is an organiclight emitting diode (OLED) pixel display device. In another embodimentof the method, the organic light emitting diode (OLED) is a smallmolecule OLED. In another embodiment of the method, the organic lightemitting diode (OLED) is a polymer OLED (PLED). In another embodiment ofthe method, the organic light emitting diode (OLED) is a phosphorescentOLED (PHOLED). In another embodiment of the method, the organic lightemitting diode (OLED) is constructed from any organic material in anycombination of single or multiple layers of organic materials andelectrodes. In another embodiment of the method, the organic lightemitting diode (OLED) is a active matrix OLED. In another embodiment ofthe method, the display device is an electroluminescent device. Inanother embodiment of the method, the display device is an plasmaemission device. In another embodiment of the method, the display deviceis any controllable photon emissive device. In another embodiment of themethod, the active matrix display device is constructed from amorphoussilicon. In another embodiment of the method, the active matrix displaydevice is constructed from poly-silicon. In another embodiment of themethod, the active matrix display device is constructed from cadmiumselenide. In another embodiment of the method, the active matrix displaydevice is constructed from any type of semiconductor material.

In another aspect the invention provides an active-matrix display andpixel structure for feedback stabilized flat panel display. In oneembodiment the invention provides an emissive pixel device having anintegrated luminance sensor, the pixel device comprising: a lightemitting device; a drive circuit generating a current to drive the lightemitting device to a predetermined luminance corresponding to an imagevoltage and applying the drive current to the light emitting deviceduring a frame time; a photo sensor that exhibits a change in electricalcharacteristic in response to a change in incident photon flux disposednear the light emitting device to intercept a measurable photon fluxwhen the light emitting device is in an emitting state; a charge storagedevice coupled with the sensor for accumulating or releasing charges andexhibiting a capacitance charge and voltage proportional to the chargeat a time; and a control circuit or other control means for controllingthe charging and discharging of the charge storage device in response tochanges in the electrical characteristics of the sensor during at leasta portion of the frame time.

In one embodiment of this device, the device further comprising: avoltage reading circuit for measuring the voltage across the chargestorage device at the end of the at least a portion of a display frametime, the measured voltage being an indication of a measured luminanceof the pixel during the portion of the frame time.

In another embodiment of the device, the device further comprising: acurrent reading circuit for measuring the current from the chargestorage device at the end of the at least a portion of a display frametime, the measured current being an indication of a measured luminanceof the pixel during the portion of the frame time.

In another embodiment of the device, the device further comprising: acharge reading circuit for measuring the charge on the charge storagedevice at the end of the at least a portion of a display frame time, themeasured charge being an indication of a measured luminance of the pixelduring the portion of the frame time.

In another embodiment of these devices, the device further comprising afeedback control circuit for applying a correction to the pixel drivecircuit during a subsequent frame time so that the measured luminanceduring the subsequent frame time will have a smaller variation from thereference luminance than during the frame time of the measurement.

In one embodiment of the device, the voltage across the charge storagedevice represents an integrated photon flux during the portion of theframe time over which the control circuit permitted charging ordischarging or the charge storage device. In another embodiment of thedevice, the voltage reading circuit further comprising a voltagecomparator circuit that receives the voltage across the charge storagedevice and a reference voltage corresponding to a target luminance andgenerates a difference signal representing the difference between thetarget luminance and the measured luminance. In another embodiment ofthe device, the current reading circuit further comprising a currentcomparator circuit that receives the current from the charge storagedevice and a reference current corresponding to a target luminance andgenerates a difference signal representing the difference between thetarget luminance and the measured luminance. In another embodiment ofthe device, the charge reading circuit further comprising a chargecomparator circuit that receives the charge on the charge storage deviceand a reference charge corresponding to a target luminance and generatesa difference signal representing the difference between the targetluminance and the measured luminance. In another embodiment of thedevice, the read circuit is configured as a charge amp/transimpedanceamplifier having a charge amplifier circuit. In another embodiment ofthe device, the charge amp/transimpedance amplifier measures the chargerequired to re-charge the storage capacitor to the full charge voltage,and that an inverting (−) input of the charge amplifier circuit has aresistance that is at least one Gig-ohm and the output of the chargeamplifier circuit has a resistance that is between about 0 ohms and 100ohms. In another embodiment of the device, the resistance of the outputof the charge amplifier circuit is a resistance that is substantiallybetween 0 ohms and 10 ohms. In another embodiment of the device, thecontrol circuit comprises at least one transistor. In another embodimentof the device, the charge storage device comprises at least onecapacitor. In another embodiment of the device, the charge storagedevice comprises multiple capacitors. In another embodiment of thedevice, the sensor device comprises a photoresistive or photoconductivedevice having a resistivity or conductivity that varies according to thenumber of photons incident on it. In another embodiment of the device,the light emitting device emits photons. In another embodiment of thedevice, the light emitting device comprises a light emitting diode. Inanother embodiment of the device, the light emitting device comprises anorganic light emitting diode. In another embodiment of the device, thelight emitting device comprises an inorganic light emitting diode. Inanother embodiment of the device, the light emitting device is one of aplurality of light emitting devices arranged as a two-dimensional arrayarranged as rows and columns. In another embodiment of the device, thelight emitting device comprises a light emitting diode.

In another embodiment of the device, the light emitting device comprisesan organic light emitting diode. In another embodiment of the device,the organic light emitting diode (OLED) is a small molecule OLED. Inanother embodiment of the device, the organic light emitting diode(OLED) is a polymer OLED (PLED). In another embodiment of the device,the organic light emitting diode (OLED) is a phosphorescent OLED(PHOLED). In another embodiment of the device, the organic lightemitting diode (OLED) is constructed from any organic material in anycombination of single or multiple layers of organic materials andelectrodes. In another embodiment of the device, the organic lightemitting diode (OLED) is a active matrix OLED. In another embodiment ofthe device, the display device is an electroluminescent device. Inanother embodiment of the device, the display device is an plasmaemission device. In another embodiment of the device, the display deviceis any controllable photon emissive device.

In another embodiment of the device, the active matrix display device isconstructed from amorphous silicon. In another embodiment of the device,the active matrix display device is constructed from poly-silicon. Inanother embodiment of the device, the active matrix display device isconstructed from cadmium selenide. In another embodiment of the device,the active matrix display device is constructed from any type ofsemiconductor material.

In another embodiment of the device, the photo sensor element includes aresistive component and the resistance changes in proportion to thephoton flux incident upon it. In another embodiment of the device, thephoto sensor element includes photodiode exhibiting a change ofresistance and/or conductance in response to photon flux incident uponit. In another embodiment of the device, the photo sensor elementincludes phototransistor exhibiting a change of resistance and/orconductance in response to photon flux incident upon it. In anotherembodiment of the device, the photo sensor intercepts photons emitted bythe light emitting device and converts them to charge carriers makingthe material of the sensor a better current conductor and thus havinglower electrical resistance. In another embodiment of the device, thelower resistance of the photo sensor drains a charge stored on acapacitor coupled in parallel across a two-terminal resistive componentof the sensor. In another embodiment of the device, the pixel circuitincludes a photon flux count integrator comprising the sensor having aresistive component and a capacitor. In another embodiment of thedevice, the amount of drained charge is proportional to the number ofphotons incident on the sensor during a portion of the frame time andthe voltage on the capacitor at the end of the portion of the frame timeis an indicator of the photons counted or integrated during the portionof the frame time.

In another embodiment of the device, a particular luminance levelproduces a photocurrent in the sensor, and the magnitude of thephotocurrent serves as an indication of the luminance (photon fluxthrough the sensor). In another embodiment of the device, thephotocurrent is proportional to the luminance. In another embodiment ofthe device, the photocurrent is directly proportional to the luminance.In another embodiment of the device, the photo responsive element isdisposed within the same pixel as the light emitting diode. In anotherembodiment of the device, the photo responsive element is integratedwith the light emitting diode so that all or substantially all thephoton flux emitted by the light emitting diode is incident on the photoresponsive element. In another embodiment of the device, the photoresponsive element has a surface or layer that is physically located incontact with a semiconductor anode side of the light emitting device.

In another embodiment of the device, the portion of the frame timecomprises the row address time or less. In another embodiment of thedevice, the portion of the frame time comprises substantially the entireframe time. In another embodiment of the device, the portion of theframe time comprises at least 50 percent of the entire frame time. Inanother embodiment of the device, the portion of the frame timecomprises at least between 90 percent and 100 percent of the entireframe time. In another embodiment of the device, the portion of theframe time comprises at least 1 millisecond. In another embodiment ofthe device, the portion of the frame time is equal to or less than therow address time. In another embodiment of the device, the portion ofthe frame time comprises a time between the row address time and theframe time. In another embodiment of this device, the portion of theframe time comprises substantially the row address time. In anotherembodiment of the device, the portion of the frame time is equal to orless than the row address time. In another embodiment of the device, theportion of the frame time is between 0.01 (1 percent) of the row addresstime and the row address time. In another embodiment of the device, theportion of the frame time is between 0.1 (10 percent) of the row addresstime and the row address time. In another embodiment of the device, theportion of the frame time is equal to or less than the frame time. Inanother embodiment of the device, the portion of the frame time isgreater than 0.01 of the row address time and less than or equal to theframe time. In another embodiment of the device, the portion of theframe time is equal to multiple frame times. In another embodiment ofthe device, the portion of the frame time comprises a time between 0.01of the row address time and the frame time.

In another aspect, the invention provides a method of operating anemissive pixel device having an integrated luminance sensor, the methodcomprising: generating a current to drive a light emitting device to apredetermined luminance corresponding to an image voltage and applyingthe drive current to the light emitting device during a frame time; acharge storage device coupled with the sensor for accumulating orreleasing charges and exhibiting a capacitance charge and voltageproportional to the charge at a time; exposing a photo sensor thatexhibits a change in electrical characteristic in response to a changein incident photon flux to photons emitted by the light emitting deviceduring the frame time; accumulating (charge) or draining (discharge)charges to or from a charge storage device coupled with the sensor, thesensor including a component that controls the rate of accumulation orrelease of charges during the frame time; measuring the voltage arisingfrom the charges present on the charge storage device at the end of aportion of the frame time, the measured voltage being an indication ofan actual luminance during the portion of the frame time; comparing theluminance related measured voltage with a reference target luminance forthe pixel emitter image voltage and pixel emitter drive current togenerate a difference value; and applying the difference value as afeedback input to a correction circuit that modifies the image voltageand drive current for the same pixel during a subsequent frame time.

In one embodiment of the method, the light emitting device comprises aninorganic light emitting diode. In one embodiment of the method, thelight emitting device comprises an organic light emitting diode (OLED).In one embodiment of the method, the organic light emitting diode (OLED)is a small molecule OLED. In one embodiment of the method, the organiclight emitting diode (OLED) is a polymer OLED (PLED). In one embodimentof the method, the organic light emitting diode (OLED) is aphosphorescent OLED (PHOLED). In one embodiment of the method, theorganic light emitting diode (OLED) is constructed from any organicmaterial in any combination of single or multiple layers of organicmaterials and electrodes. In one embodiment of the method, the organiclight emitting diode (OLED) is a active matrix OLED. In one embodimentof the method, the display emissive device is an electroluminescentdevice. In one embodiment of the method, the display emissive device isan plasma emission device. In one embodiment of the method, the displayemissive device is any controllable photon emissive device.

In one embodiment of the method, the active matrix is constructed fromamorphous silicon. In one embodiment of the method, the active matrix isconstructed from poly silicon. In one embodiment of the method, theactive matrix is constructed from cadmium selenide. In one embodiment ofthe method, the active matrix is constructed from any type ofsemiconductor material.

In one embodiment of the method, the photo sensor intercepts photonsemitted by the light emitting device and converts them to chargecarriers making the material of the sensor a better current conductorand thus having lower electrical resistance. In one embodiment of themethod, the amount of accumulated or drained charge is proportional tothe number of photons incident on the sensor during a portion of theframe time and the voltage on the capacitor at the end of the portion ofthe frame time is an indicator of the photons counted or integratedduring the portion of the frame time. In one embodiment of the method, aparticular luminance level produces a photocurrent in the sensor, andthe magnitude of the photocurrent serves as an indication of theluminance (photon flux through the sensor). In one embodiment of themethod, the photo sensor element is disposed within the same pixel asthe light emitting diode.

In one embodiment of the method, the portion of the frame time comprisesthe row address time or less. In one embodiment of the method, theportion of the frame time comprises substantially the entire frame time.In one embodiment of the method, the portion of the frame time comprisesat least 50 percent of the entire frame time. In one embodiment of themethod, the portion of the frame time comprises at least between 90percent and 100 percent of the entire frame time. In one embodiment ofthe method, the portion of the frame time comprises at least 1millisecond. In one embodiment of the method, the portion of the frametime is equal to or less than the row address time.

In another embodiment of the method, the portion of the frame timecomprises a time between the row address time and the frame time. Inanother embodiment of this method, the portion of the frame timecomprises substantially the row address time. In another embodiment ofthe method, the portion of the frame time is equal to or less than therow address time. In another embodiment of the method, the portion ofthe frame time is between 0.01 (1 percent) of the row address time andthe row address time. In another embodiment of the method, the portionof the frame time is between 0.1 (10 percent) of the row address timeand the row address time. In another embodiment of the method, theportion of the frame time is equal to or less than the frame time. Inanother embodiment of the method, the portion of the frame time isgreater than 0.01 of the row address time and less than or equal to theframe time. In another embodiment of the method, the portion of theframe time is equal to multiple frame times. In another embodiment ofthe method, the portion of the frame time comprises a time between 0.01of the row address time and the frame time.

In another aspect the invention provides a device and method foroperating a self-calibrating emissive pixel. In one embodiment theinvention provides an emissive pixel device and a method for operating aself-calibrating pixel, the method comprising: establishing a sensorcapacitor at a predetermined starting voltage; delivering a current to aphoton emitting device to cause photons to be emitted at a predeterminedtarget photon emission level; exposing a sensor device, havingelectrical properties that varies according to a photon flux on thesensor device, to the emitted photon emission during at least a portionof a display frame time; permitting the sensor capacitor to eithercharge or discharge from the predetermined starting voltage through thesensor device so that the portion of the frame time and the averageresistance of the sensor during the portion of the frame time determinethe amount of charge on the sensor capacitor; measuring the voltage orcharge remaining on the sensor capacitor at the end of a portion of theframe time as an indication of the integrated photon flux and pixelluminance during the portion of the frame time used for measurement; andmodifying the image voltage and current to be applied to the same pixeland gray level during a subsequent display frame time using the measuredsensor capacitor voltage as a feedback parameter.

In one embodiment of this method, the sensor comprises a photoresistivedevice. In one embodiment of this method, the sensor comprises aphotoconductive device. In one embodiment of this method, the sensorcomprises at least one of a photodiode, a photoresistor, aphotoconductor, and a phototransistor. In one embodiment of this method,the sensor comprises a phototransistor. In one embodiment of thismethod, the sensor comprises a photodiode. In one embodiment of thismethod, the established capacitor starting voltage is established bycharging the sensor capacitor to a predetermined charging voltage. Inone embodiment of this method, the established capacitor startingvoltage is established at substantially zero volts. In one embodiment ofthis method, the predetermined capacitor starting voltage is a non-zerovoltage having a voltage magnitude. In one embodiment of this method,for a sensor capacitor that was charged to a non-zero predeterminedstarting voltage and then permitted to discharge, the difference voltageremaining across the sensor capacitor is an indication of total photonintegrated flux during the portion of the frame time.

In one embodiment of this method, for a sensor capacitor that wasuncharged at substantially zero volts or charged at a different voltageand then permitted to charge during the portion of the frame integrationtime, the difference of the starting voltage and the ending voltageacross the sensor capacitor is an indication of total photon integratedflux during the portion of the frame time.

In one embodiment of this method, the step of modifying the imagevoltage and current to be applied to the same pixel and gray levelduring a subsequent display frame further comprises comparing themeasured sensor capacitor voltage with a reference calibration voltagestored in a memory and generating a correction using the differencebetween these voltages.

In one embodiment of this method, the method is performed substantiallyin parallel for each pixel of a two-dimensional active-matrix pixelarray.

In one embodiment of this method, the current delivered is delivered byapplying a voltage to a control device that delivers a currentcorresponding to that voltage to the photon emitting device to causephotons to be emitted at a predetermined target photon emission level.

In one embodiment of this method, the portion of the frame timecomprises the row address time or less. In one embodiment of thismethod, the portion of the frame time comprises substantially the entireframe time. In one embodiment of this method, the portion of the frametime comprises at least 50 percent of the entire frame time. In oneembodiment of this method, the portion of the frame time comprises atleast between 90 percent and 100 percent of the entire frame time. Inone embodiment of this method, the portion of the frame time comprisesat least 1 millisecond. In one embodiment of this method, the portion ofthe frame time is equal to or less than the row address time. In anotherembodiment of the method, the portion of the frame time is between 0.01(1 percent) of the row address time and the row address time. In anotherembodiment of the method, the portion of the frame time is between 0.1(10 percent) of the row address time and the row address time. Inanother embodiment of the method, the portion of the frame time is equalto or less than the frame time. In another embodiment of the method, theportion of the frame time is greater than 0.01 of the row address timeand less than or equal to the frame time. In another embodiment of themethod, the portion of the frame time is equal to multiple frame times.In another embodiment of the method, the portion of the frame timecomprises a time between 0.01 of the row address time and the frametime.

In one embodiment of the method, the method further comprising charginga sensor coupled capacitor to a first predetermined voltage through asensor line by a transistor and capacitor charging voltage source priorto an integration frame time. In one embodiment of the method, acapacitor charge voltage is applied over a sensor line and the sensorline only delivers current when a measurement is being made of thesensor capacitor voltage or when sensor capacitor is being recharged andthe voltage is highly stable and not subject to variation.

In another aspect the invention provides high-performance emissivedisplay device for computers, information appliances, and entertainmentsystems. In one embodiment the invention provides an informationappliance comprising: a flat panel or other display device comprising aplurality of active-matrix pixels arranged as a two-dimensional array,each pixel including an organic light emitting diode emitter, an emitterdrive circuit receiving an input image data for each pixel andgenerating a pixel drive signal intended to produce a correspondingtarget pixel luminance during a frame time, and an emitter luminancesensor and measurement circuit that measures an electrical parameterindicative of the actual luminance of each pixel over a portion of ameasurement display frame time; and a display logic subsystem coupled tothe flat panel display device and receiving the pixel luminance relatedelectrical parameter for each pixel and generating a correction to beapplied during a frame time subsequent to the measurement display frametime to the input image data for each pixel based on a differencebetween the target pixel luminance and the measured pixel luminance.

In one embodiment, the information appliance further comprises at leastone of: a television monitor, a television receiver, a CD player, a DVDplayer, a computer monitor or display, a computer system, an automobileinstrument panel, an aircraft instrument display panel, a video game, acellular telephone, a personal data assistant (PDA), a telephone, agraphics system, a printing system, a scoreboard system, anentertainment system, a domestic or home appliance, a copy machine, aglobal positioning system navigation display, a dynamic art displaydevice, a camera, and any combinations thereof.

In one embodiment of this information appliance, each of the pixelscomprises: a light emitting device; a drive circuit generating a currentto drive the light emitting device to a predetermined luminancecorresponding to an image voltage and applying the drive current to thelight emitting device during a frame time; a photo sensor that exhibitsa change in electrical characteristic in response to a change inincident photon flux disposed near the light emitting device tointercept a measurable photon flux when the light emitting device is inan emitting state; a charge storage device coupled with the sensor foraccumulating or releasing charges and exhibiting a capacitance chargeand voltage proportional to the charge at a time; a control circuitcontrolling the charging and discharging of the charge storage device inresponse to changes in the electrical characteristics of the sensorduring at least a portion of the frame time; a voltage reading circuitfor measuring the voltage across the charge storage device at the end ofthe at least a portion of a display frame time, the measured voltagebeing an indication of a measured luminance of the pixel during theportion of the frame time; and a feedback control circuit for applying acorrection to the pixel drive circuit during a subsequent frame time sothat the measured luminance during the subsequent frame time will have asmaller variation from the reference luminance than during the frametime of the measurement.

In another embodiment, the invention provides a method of operating adisplay device of the type having a plurality of active-matrix pixelsarranged as a two-dimensional array, each pixel including a lightemitting diode emitter and an emitter drive circuit receiving an inputimage data for each pixel and generating a pixel drive signal intendedto produce a corresponding target pixel luminance during each framedisplay time; the method characterized in that the method furtherincludes: measuring a voltage indicative of a photon flux intercepted byan emitter luminance measurement circuit during at least a portion of afirst frame time; and comparing the measured voltage corresponding to ameasured luminance with a reference voltage corresponding to a referenceluminance to generate a difference signal and using the differencesignal to modify the input image data for each pixel during a subsequentframe display time so that the pixel luminance during the subsequentdisplay frame time will more nearly equal the reference luminance.

In one embodiment of this method, the portion of the frame timecomprises the row address time or less. In one embodiment of thismethod, the portion of the frame display time comprises substantiallythe entire frame time. In one embodiment of this method, the portion ofthe frame display time comprises at least 50 percent of the entire frametime. In one embodiment of this method, the portion of the frame displaytime comprises at least between 90 percent and 100 percent of the entireframe time. In one embodiment of this method, the portion of the framedisplay time comprises at least 1 millisecond. In one embodiment of thismethod, the portion of the frame time is equal to or less than the rowaddress time. In another embodiment of the method, the portion of theframe time is between 0.01 (1 percent) of the row address time and therow address time. In another embodiment of the method, the portion ofthe frame time is between 0.1 (10 percent) of the row address time andthe row address time. In another embodiment of the method, the portionof the frame time is equal to or less than the frame time. In anotherembodiment of the method, the portion of the frame time is greater than0.01 of the row address time and less than or equal to the frame time.In another embodiment of the method, the portion of the frame time isequal to multiple frame times. In another embodiment of the method, theportion of the frame time comprises a time between 0.01 of the rowaddress time and the frame time.

In one embodiment of the method, the subsequent frame display time is aframe display immediately following the first display time. In oneembodiment of the method, the subsequent frame display time is a framedisplay a predetermined number of display frames following the firstframe display time for which the luminance measurement was made, andwherein the predetermined number of frames is any integer number offrames N. In one embodiment of the method, the subsequent frame displaytime is a frame display at the occurrence of a predetermined ordynamically determined event.

In one embodiment of the method, the occurrence of a predetermined ordynamically determined event is selected from a display initializationevent, a display power-on event, a display time of operation event, auser initiated event, any automatic policy or rule based event, andcombinations of these.

In one embodiment of the method, the display device comprises a flatpanel display device that is a component in an overall system andwherein the system is selected from the set of systems consisting of:any information appliance, a television monitor, a CD player, a DVDplayer, a computer monitor, a computer system, an automobile instrumentpanel, an aircraft instrument display panel, a video game, a cellulartelephone, a personal data assistant (PDA), a telephone, a graphicssystem, a printing system, a scoreboard system, an entertainment system,a domestic or home appliance, a copy machine, a global positioningsystem navigation display, a dynamic art display device, a camera, andany combinations thereof.

In one embodiment of the appliance and method, the light emitting devicecomprises an organic light emitting diode (OLED). In one embodiment ofthe appliance and method, the organic light emitting diode (OLED) is asmall molecule OLED. In one embodiment of the appliance and method, theorganic light emitting diode (OLED) is a polymer OLED (PLED). In oneembodiment of the appliance and method, the organic light emitting diode(OLED) is a phosphorescent OLED (PHOLED). In one embodiment of theappliance and method, the organic light emitting diode (OLED) isconstructed from any organic material in any combination of single ormultiple layers of organic materials and electrodes. In one embodimentof the appliance and method, the organic light emitting diode (OLED) isa active matrix OLED. In one embodiment of the appliance and method, thelight emitting device is an electroluminescent device. In one embodimentof the appliance and method, the light emitting device is a plasmaemission device. In one embodiment of the appliance and method, thelight emitting device is any controllable photon emissive device.

In one embodiment of the appliance and method, the display device isconstructed from amorphous silicon. In one embodiment of the applianceand method, the display device is constructed from poly silicon. In oneembodiment of the appliance and method, the display device isconstructed from cadmium selenide. In one embodiment of the applianceand method, the display device is constructed from any type ofsemiconductor material.

In another aspect, the invention provides an integrated circuit. In oneembodiment, the integrated circuit comprises: a sample and hold circuitreceiving an analog voltage signals characterizing integrated photonflux and luminance measurements from a plurality of display pixels; ananalog-to-digital converter receiving the sampled and held analogvoltage signal and converting the analog signal to a digital signal; acalibration value memory for storing a reference value for each pixeland for each gray level value the pixel may be required to display; atleast one comparator receiving at least one of the converted digitalsignal value indicating a particular measured pixel luminance and atleast one reference signal value indicating a reference luminance forthe same pixel and generating a difference signal indicating a deviationof the measured pixel luminance from the reference pixel luminance; anda pixel deviation logic including a pixel deviation memory for storingan indication of the deviation for the pixel. In another embodiment ofthe integrated circuit, the pixel deviation memory and the calibrationvalue memory are logically defined within a common physical memory. Inanother embodiment of the integrated circuit, the pixel deviation memoryand the calibration value memory are defined within different physicalmemories.

Having described several methods in considerable detail it will beappreciated that these descriptions include optional device, apparatus,system, and methodological steps (features) that may be combined so thatfewer than the recited number of features may be implemented to achievethe same or substantially same result. It will also be appreciated thatthe order of the steps in method claims may be modified in manyinstances to achieve the same or substantially the same results and thatthe connectivity of circuits and devices may often be modified whilestill achieving the performance of the invention.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention.

1. A method for operating a self-calibrating pixel, the methodcomprising: establishing a sensor capacitor at a predetermined startingvoltage; delivering a current to a photon emitting device to causephotons to be emitted at a predetermined target photon emission level;exposing a sensor device, having electrical properties that variesaccording to a photon flux on the sensor device, to the emitted photonemission during at least a portion of a display frame time; permittingthe sensor capacitor to either charge or discharge from thepredetermined starting voltage through the sensor device so that theportion of the frame time and the average resistance of the sensorduring the portion of the frame time determine the amount of charge onthe sensor capacitor; measuring the voltage or charge remaining on thesensor capacitor at the end of a portion of the frame time as anindication of the integrated photon flux and pixel luminance during theportion of the frame time used for measurement; and modifying the imagevoltage and current to be applied to the same pixel and gray levelduring a subsequent display frame time using the measured sensorcapacitor voltage as a feedback parameter.
 2. A method as in claim 1,wherein the sensor comprises a photoresistive device.
 3. A method as inclaim 1, wherein the sensor comprises a photoconductive device.
 4. Amethod as in claim 1, wherein the sensor comprises at least one of aphotodiode, a photoresistor, a photoconductor, and a phototransistor. 5.A method as in claim 1, wherein the sensor comprises a phototransistor.6. A method as in claim 1, wherein the sensor comprises a photodiode. 7.A method as in claim 1, wherein the established capacitor startingvoltage is established by charging the sensor capacitor to apredetermined charging voltage.
 8. A method as in claim 1, wherein theestablished capacitor starting voltage is established at substantiallyzero volts.
 9. A method as in claim 1, wherein the predeterminedcapacitor starting voltage is a non-zero voltage having a voltagemagnitude.
 10. A method as in claim 1, wherein for a sensor capacitorthat was charged to a non-zero predetermined starting voltage and thenpermitted to discharge, the difference voltage remaining across thesensor capacitor is an indication of total photon integrated flux duringthe portion of the frame time.
 11. A method as in claim 1, wherein for asensor capacitor that was uncharged at substantially zero volts orcharged at a different voltage and then permitted to charge during theportion of the frame integration time, the difference of the startingvoltage and the ending voltage across the sensor capacitor is anindication of total photon integrated flux during the portion of theframe time.
 12. A method as in claim 1, wherein the step of modifyingthe image voltage and current to be applied to the same pixel and graylevel during a subsequent display frame further comprises comparing themeasured sensor capacitor voltage with a reference calibration voltagestored in a memory and generating a correction using the differencebetween these voltages.
 13. A method as in claim 1, wherein the methodis performed substantially in parallel for each pixel of atwo-dimensional active-matrix pixel array.
 14. A method as in claim 1,wherein the current delivered is delivered by applying a voltage to acontrol device that delivers a current corresponding to that voltage tothe photon emitting device to cause photons to be emitted at apredetermined target photon emission level.
 15. A method as in claim 1,wherein the portion of the frame time comprises the row address time orless.
 16. A method as in claim 1, wherein the portion of the frame timecomprises substantially the entire frame time.
 17. A method as in claim1, wherein the portion of the frame time comprises at least 50 percentof the entire frame time.
 18. A method as in claim 1, wherein theportion of the frame time comprises at least between 90 percent and 100percent of the entire frame time.
 19. A method as in claim 1, whereinthe portion of the frame time comprises at least 1 millisecond.
 20. Amethod as in claim 1, wherein the portion of the frame time is equal toor less than the row address time.
 21. A method as in claim 1, whereinthe method further comprising charging a sensor coupled capacitor to afirst predetermined voltage through a sensor line by a transistor andcapacitor charging voltage source prior to an integration frame time.22. A method as in claim 21, wherein a capacitor charge voltage isapplied over a sensor line and the sensor line only delivers currentwhen a measurement is being made of the sensor capacitor voltage or whensensor capacitor is being recharged and the voltage is highly stable andnot subject to variation.